Re: kernel 2.6.13 buffer strangeness

Re: kernel 2.6.13 buffer strangeness

[Anthony Wesley]

Thanks David, but if you read my original post in full you'll see that I've tried that, and while I can start the write out sooner by lowering /proc/sys/vm/dirty_ratio , it makes no difference to the results that I am getting. I still seem to run out of steam after only 50 seconds where it should take about 3 minutes.

regards, Anthony

-- 
Anthony Wesley
Director and IT/Network Consultant
Smart Networks Pty Ltd
Acquerra Pty Ltd

Anthony.Wesley@acquerra.com.au
Phone: (02) 62595404 or 0419409836

RE: kernel 2.6.13 buffer strangeness

[Roger Heflin]

I saw it mentioned before that the kernel only allows a certain
percentage of total memory to be dirty, I thought the number was
around 40%, and I have seen machines with large amounts of ram,
hit the 40% and then put the writing application into disk wait
until certain amounts of things are written out, and then take
it out of disk wait, and repeat when it again hits 40%, given your
rate different it would be close to 40% in 50seconds.

And I think that you mean MB(yte) not Mb(it).

                           Roger

> -----Original Message-----
> From: linux-kernel-owner@vger.kernel.org 
> [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of 
> Anthony Wesley
> Sent: Friday, September 09, 2005 4:11 AM
> To: linux-kernel@vger.kernel.org
> Subject: Re: kernel 2.6.13 buffer strangeness
> 
> Thanks David, but if you read my original post in full you'll 
> see that I've tried that, and while I can start the write out 
> sooner by lowering /proc/sys/vm/dirty_ratio , it makes no 
> difference to the results that I am getting. I still seem to 
> run out of steam after only 50 seconds where it should take 
> about 3 minutes.
> 
> regards, Anthony
> 
> --
> Anthony Wesley
> Director and IT/Network Consultant
> Smart Networks Pty Ltd
> Acquerra Pty Ltd
> 
> Anthony.Wesley@acquerra.com.au
> Phone: (02) 62595404 or 0419409836
> 
> -
> To unsubscribe from this list: send the line "unsubscribe 
> linux-kernel" in the body of a message to 
> majordomo@vger.kernel.org More majordomo info at  
> http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 

Re: kernel 2.6.13 buffer strangeness

[Nate Diller]

On 9/9/05, Roger Heflin <rheflin@atipa.com> wrote:
> 
> I saw it mentioned before that the kernel only allows a certain
> percentage of total memory to be dirty, I thought the number was
> around 40%, and I have seen machines with large amounts of ram,
> hit the 40% and then put the writing application into disk wait
> until certain amounts of things are written out, and then take
> it out of disk wait, and repeat when it again hits 40%, given your
> rate different it would be close to 40% in 50seconds.
> 
yes, on 2.6 there are two tunables which are important here. 
dirty_background_ratio is the threshold where the kernel will begin
flushing dirty buffers, so it should change how soon the disk becomes
active.  dirty_ratio changes when the write-throttling code kicks in,
which is what Anthony is seeing.  The purpose of the write throttling
code is to limit the dirtying process to disk bandwidth, so that is a
Feature.  Anthony, try *increasing* dirty_ratio, you can go up to 100,
but you could trigger an OOM if you let it get too high, so maybe try
setting it at 85 or so.  This should effectively disable the write
throttling and give you the bandwidth you want.

NATE

> And I think that you mean MB(yte) not Mb(it).
> 
>                            Roger
> 
> > -----Original Message-----
> > From: linux-kernel-owner@vger.kernel.org 
> > [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of 
> > Anthony Wesley
> > Sent: Friday, September 09, 2005 4:11 AM
> > To: linux-kernel@vger.kernel.org
> > Subject: Re: kernel 2.6.13 buffer strangeness
> > 
> > Thanks David, but if you read my original post in full you'll 
> > see that I've tried that, and while I can start the write out 
> > sooner by lowering /proc/sys/vm/dirty_ratio , it makes no 
> > difference to the results that I am getting. I still seem to 
> > run out of steam after only 50 seconds where it should take 
> > about 3 minutes.
> > 
> > regards, Anthony
> > 
> > --
> > Anthony Wesley
> > Director and IT/Network Consultant
> > Smart Networks Pty Ltd
> > Acquerra Pty Ltd
> > 
> > Anthony.Wesley@acquerra.com.au
> > Phone: (02) 62595404 or 0419409836
> > 
> > -
> > To unsubscribe from this list: send the line "unsubscribe 
> > linux-kernel" in the body of a message to 
> > majordomo@vger.kernel.org More majordomo info at  
> > http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/
> > 
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
>

Re: kernel 2.6.13 buffer strangeness

[Anthony Wesley]

Hi Nate and Roger,

First off, thanks fo taking some time to answer my call for help :-)

I've already spent some time fiddling with dirty_ratio before I posted my question - yes, I can make different things happen by increasing or decreasing it, but overall I still have the same problem - my video streaming runs out of steam after at most 50 seconds instead of the 3 minutes that I think it should take.

Setting dirty_ratio and dirty_background_ratio to 15/10 respectively makes things worse, I hit the wall after only about 25 seconds.

Setting dirty_ratio and dirty_background_ratio to 85/80 makes things worse - disk writes start after about 15 seconds and I hit the wall after about 35 seconds.

Setting dirty_ratio and dirty_background_ratio to 90/10 puts me back at around 50 seconds, i.e. where I started.

So as far as I can see there is *no way* to get 3 minutes worth of buffering by adjusting these parameters.

Just to remind everyone - I have video data coming in at 25MBytes/sec and I am writing it out to a usb2 hard disk that can sustain 17MBytes/sec. I want my video capture to run at full speed as long as possible by having the 7MBytes/sec deficit slowly eat up the available RAM in the machine. I have 1.5Gb of RAM, 1.3Gb available for buffers, so this should take 3 minutes to consume at 7MBytes/sec.

So, I've tried all the combinations on dirty_ratio and dirty_background_ratio and they *do not help*.

Can anyone suggest something else that I might try? The goal is to have 25MBytes/sec streaming video for about 3 minutes. 

Or is this simply not possible with the current kernel I/O setup? Do I have to do something elaborate myself, like build a big RAM buffer, mount the disk synchronous, do the buffering myself in userland...??

regards, Anthony

Nate Diller wrote:

> On 9/9/05, Roger Heflin <rheflin@atipa.com> wrote:
> 
>>I saw it mentioned before that the kernel only allows a certain
>>percentage of total memory to be dirty, I thought the number was
>>around 40%, and I have seen machines with large amounts of ram,
>>hit the 40% and then put the writing application into disk wait
>>until certain amounts of things are written out, and then take
>>it out of disk wait, and repeat when it again hits 40%, given your
>>rate different it would be close to 40% in 50seconds.
>>
> 
> yes, on 2.6 there are two tunables which are important here. 
> dirty_background_ratio is the threshold where the kernel will begin
> flushing dirty buffers, so it should change how soon the disk becomes
> active.  dirty_ratio changes when the write-throttling code kicks in,
> which is what Anthony is seeing.  The purpose of the write throttling
> code is to limit the dirtying process to disk bandwidth, so that is a
> Feature.  Anthony, try *increasing* dirty_ratio, you can go up to 100,
> but you could trigger an OOM if you let it get too high, so maybe try
> setting it at 85 or so.  This should effectively disable the write
> throttling and give you the bandwidth you want.
> 
> NATE
> 
> 
>>And I think that you mean MB(yte) not Mb(it).
>>
>>                           Roger
>>
>>
>>>-----Original Message-----
>>>From: linux-kernel-owner@vger.kernel.org 
>>>[mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of 
>>>Anthony Wesley
>>>Sent: Friday, September 09, 2005 4:11 AM
>>>To: linux-kernel@vger.kernel.org
>>>Subject: Re: kernel 2.6.13 buffer strangeness
>>>
>>>Thanks David, but if you read my original post in full you'll 
>>>see that I've tried that, and while I can start the write out 
>>>sooner by lowering /proc/sys/vm/dirty_ratio , it makes no 
>>>difference to the results that I am getting. I still seem to 
>>>run out of steam after only 50 seconds where it should take 
>>>about 3 minutes.
>>>
>>>regards, Anthony
>>>
>>>--
>>>Anthony Wesley
>>>Director and IT/Network Consultant
>>>Smart Networks Pty Ltd
>>>Acquerra Pty Ltd
>>>
>>>Anthony.Wesley@acquerra.com.au
>>>Phone: (02) 62595404 or 0419409836
>>>
>>>-
>>>To unsubscribe from this list: send the line "unsubscribe 
>>>linux-kernel" in the body of a message to 
>>>majordomo@vger.kernel.org More majordomo info at  
>>>http://vger.kernel.org/majordomo-info.html
>>>Please read the FAQ at  http://www.tux.org/lkml/
>>>
>>
>>-
>>To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>>the body of a message to majordomo@vger.kernel.org
>>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>Please read the FAQ at  http://www.tux.org/lkml/
>>
> 
> 

-- 
Anthony Wesley
Director and IT/Network Consultant
Smart Networks Pty Ltd
Acquerra Pty Ltd

Anthony.Wesley@acquerra.com.au
Phone: (02) 62595404 or 0419409836

Re: kernel 2.6.13 buffer strangeness

[Nate Diller]

On 9/9/05, Anthony Wesley <awesley@acquerra.com.au> wrote:
> Hi Nate and Roger,
> 
> First off, thanks fo taking some time to answer my call for help :-)
> 
> I've already spent some time fiddling with dirty_ratio before I posted my
> question - yes, I can make different things happen by increasing or
> decreasing it, but overall I still have the same problem - my video
> streaming runs out of steam after at most 50 seconds instead of the 3
> minutes that I think it should take.
> 
> Setting dirty_ratio and dirty_background_ratio to 15/10 respectively makes
> things worse, I hit the wall after only about 25 seconds.
> 
not surprising

> Setting dirty_ratio and dirty_background_ratio to 85/80 makes things worse -
> disk writes start after about 15 seconds and I hit the wall after about 35
> seconds.
> 
so dirty_background_ratio is working, that's why it took a long time
to start writing anything out

> Setting dirty_ratio and dirty_background_ratio to 90/10 puts me back at
> around 50 seconds, i.e. where I started.
> 
this setting should do the trick, so there's something going on here
that isn't expected.

> So as far as I can see there is *no way* to get 3 minutes worth of buffering
> by adjusting these parameters.
> 
> Just to remind everyone - I have video data coming in at 25MBytes/sec and I
> am writing it out to a usb2 hard disk that can sustain 17MBytes/sec. I want
> my video capture to run at full speed as long as possible by having the
> 7MBytes/sec deficit slowly eat up the available RAM in the machine. I have
> 1.5Gb of RAM, 1.3Gb available for buffers, so this should take 3 minutes to
> consume at 7MBytes/sec.
> 
> So, I've tried all the combinations on dirty_ratio and
> dirty_background_ratio and they *do not help*.
> 
dirty_ratio is the tubable you want, if it's not working correctly,
either there's a problem with your setup, or a bug

> Can anyone suggest something else that I might try? The goal is to have
> 25MBytes/sec streaming video for about 3 minutes. 
> 
how sure are you that you're getting 17MB/s during this test?  can you
run "vmstat 1" while this is running to verify?  which FS and
scheduler?

just for interest, what's the raw disk bandwidth (use hdparm, or run a
dd, or something)?  it would obviously be much better to sustain
25MB/s to disk

> Or is this simply not possible with the current kernel I/O setup? Do I have
> to do something elaborate myself, like build a big RAM buffer, mount the
> disk synchronous, do the buffering myself in userland...??
> 
this should be possible, although it could be considered a bit risky WRT OOM.

NATE

> regards, Anthony
> 
> Nate Diller wrote:
> 
> > On 9/9/05, Roger Heflin <rheflin@atipa.com> wrote:
> > 
> >>I saw it mentioned before that the kernel only allows a certain
> >>percentage of total memory to be dirty, I thought the number was
> >>around 40%, and I have seen machines with large amounts of ram,
> >>hit the 40% and then put the writing application into disk wait
> >>until certain amounts of things are written out, and then take
> >>it out of disk wait, and repeat when it again hits 40%, given your
> >>rate different it would be close to 40% in 50seconds.
> >>
> > 
> > yes, on 2.6 there are two tunables which are important here. 
> > dirty_background_ratio is the threshold where the kernel will begin
> > flushing dirty buffers, so it should change how soon the disk becomes
> > active.  dirty_ratio changes when the write-throttling code kicks in,
> > which is what Anthony is seeing.  The purpose of the write throttling
> > code is to limit the dirtying process to disk bandwidth, so that is a
> > Feature.  Anthony, try *increasing* dirty_ratio, you can go up to 100,
> > but you could trigger an OOM if you let it get too high, so maybe try
> > setting it at 85 or so.  This should effectively disable the write
> > throttling and give you the bandwidth you want.
> > 
> > NATE
> > 
> > 
> >>And I think that you mean MB(yte) not Mb(it).
> >>
> >>                           Roger
> >>
> >>
> >>>-----Original Message-----
> >>>From: linux-kernel-owner@vger.kernel.org 
> >>>[mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of 
> >>>Anthony Wesley
> >>>Sent: Friday, September 09, 2005 4:11 AM
> >>>To: linux-kernel@vger.kernel.org
> >>>Subject: Re: kernel 2.6.13 buffer strangeness
> >>>
> >>>Thanks David, but if you read my original post in full you'll 
> >>>see that I've tried that, and while I can start the write out 
> >>>sooner by lowering /proc/sys/vm/dirty_ratio , it makes no 
> >>>difference to the results that I am getting. I still seem to 
> >>>run out of steam after only 50 seconds where it should take 
> >>>about 3 minutes.
> >>>
> >>>regards, Anthony
> >>>
> >>>--
> >>>Anthony Wesley
> >>>Director and IT/Network Consultant
> >>>Smart Networks Pty Ltd
> >>>Acquerra Pty Ltd
> >>>
> >>>Anthony.Wesley@acquerra.com.au
> >>>Phone: (02) 62595404 or 0419409836
> >>>
> >>>-
> >>>To unsubscribe from this list: send the line "unsubscribe 
> >>>linux-kernel" in the body of a message to 
> >>>majordomo@vger.kernel.org More majordomo info at  
> >>>http://vger.kernel.org/majordomo-info.html
> >>>Please read the FAQ at  http://www.tux.org/lkml/
> >>>
> >>
> >>-
> >>To unsubscribe from this list: send the line "unsubscribe linux-kernel"
> in
> >>the body of a message to majordomo@vger.kernel.org
> >>More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >>Please read the FAQ at  http://www.tux.org/lkml/
> >>
> > 
> > 
> 
> -- 
> Anthony Wesley
> Director and IT/Network Consultant
> Smart Networks Pty Ltd
> Acquerra Pty Ltd
> 
> Anthony.Wesley@acquerra.com.au
> Phone: (02) 62595404 or 0419409836
>

Re: kernel 2.6.13 buffer strangeness

[Anthony Wesley]

Okay, just tested a couple of things, here's what I see...

I tested the write speed to the usb2 hard disk and got 21MBytes/sec. It's a laptop 
hard drive, only 5400rpm so this is not surprising.

I did this test with my video capture app running, but just displaying data and not writing,

I have another laptop usb2 hard drive here which I just tested and got 17MBytes/sec - it's
a 4200rpm drive so again not surprising numbers.

The video data is written as individual frames, so the efficiency is a bit below the raw throughput,
but my tests were transferring 1.5Gb of data as raw frames - exactly the same way that Coriander would
write its data.

I'd bet a large sum of money that these hard disk figures are correct to within a few percent.

Also, when I am actually capturing I have timed by stopwatch how long the disk activity light
is on, and reached about the same conclusion.

Working the problem from the other direction, the only way to explain the early throttling that
I see would be if *almost no data* is being written to the disk, and this is plainly not the case. Even if
the disk were running at a greatly reduced rate of (say) 10MBytes/sec I would still see 86 seconds
of buffering before the throttle kicks in, and so far I have managed only to get to about 65 or 70.

regards, Anthony

Nate Diller wrote:


>>Setting dirty_ratio and dirty_background_ratio to 90/10 puts me back at
>>around 50 seconds, i.e. where I started.
>>
> 
> this setting should do the trick, so there's something going on here
> that isn't expected.
> 
> 
>>So as far as I can see there is *no way* to get 3 minutes worth of buffering
>>by adjusting these parameters.
>>
>>Just to remind everyone - I have video data coming in at 25MBytes/sec and I
>>am writing it out to a usb2 hard disk that can sustain 17MBytes/sec. I want
>>my video capture to run at full speed as long as possible by having the
>>7MBytes/sec deficit slowly eat up the available RAM in the machine. I have
>>1.5Gb of RAM, 1.3Gb available for buffers, so this should take 3 minutes to
>>consume at 7MBytes/sec.
>>
>>So, I've tried all the combinations on dirty_ratio and
>>dirty_background_ratio and they *do not help*.
>>
> 
> dirty_ratio is the tubable you want, if it's not working correctly,
> either there's a problem with your setup, or a bug
> 
> 
>>Can anyone suggest something else that I might try? The goal is to have
>>25MBytes/sec streaming video for about 3 minutes. 
>>
> 
> how sure are you that you're getting 17MB/s during this test?  can you
> run "vmstat 1" while this is running to verify?  which FS and
> scheduler?
> 
> just for interest, what's the raw disk bandwidth (use hdparm, or run a
> dd, or something)?  it would obviously be much better to sustain
> 25MB/s to disk
> 
> 
>>Or is this simply not possible with the current kernel I/O setup? Do I have
>>to do something elaborate myself, like build a big RAM buffer, mount the
>>disk synchronous, do the buffering myself in userland...??
>>
> 
> this should be possible, although it could be considered a bit risky WRT OOM.
> 
> NATE

-- 
Anthony Wesley
Director and IT/Network Consultant
Smart Networks Pty Ltd
Acquerra Pty Ltd

Anthony.Wesley@acquerra.com.au
Phone: (02) 62595404 or 0419409836

Re: kernel 2.6.13 buffer strangeness

[Nate Diller]

On 9/9/05, Anthony Wesley <awesley@acquerra.com.au> wrote:
> Okay, just tested a couple of things, here's what I see...
> 
> I tested the write speed to the usb2 hard disk and got 21MBytes/sec. It's a
> laptop 
> hard drive, only 5400rpm so this is not surprising.
> 
> I did this test with my video capture app running, but just displaying data
> and not writing,
> 
> I have another laptop usb2 hard drive here which I just tested and got
> 17MBytes/sec - it's
> a 4200rpm drive so again not surprising numbers.
> 
> The video data is written as individual frames, so the efficiency is a bit
> below the raw throughput,
> but my tests were transferring 1.5Gb of data as raw frames - exactly the
> same way that Coriander would
> write its data.
> 
> I'd bet a large sum of money that these hard disk figures are correct to
> within a few percent.
> 
> Also, when I am actually capturing I have timed by stopwatch how long the
> disk activity light
> is on, and reached about the same conclusion.
> 
> Working the problem from the other direction, the only way to explain the
> early throttling that
> I see would be if *almost no data* is being written to the disk, and this is
> plainly not the case. Even if
> the disk were running at a greatly reduced rate of (say) 10MBytes/sec I
> would still see 86 seconds
> of buffering before the throttle kicks in, and so far I have managed only to
> get to about 65 or 70.
> 
> regards, Anthony
> 
i need a vmstat trace ('vmstat 1 > logfile', then run the test, then
^c)  before i can comment further.  also, your filesystem and
scheduler would be interesting to know.  you can find out the
scheduler with 'cat /sys/block/[drive]/queue/scheduler'.

NATE

> Nate Diller wrote:
> 
> 
> >>Setting dirty_ratio and dirty_background_ratio to 90/10 puts me back at
> >>around 50 seconds, i.e. where I started.
> >>
> > 
> > this setting should do the trick, so there's something going on here
> > that isn't expected.
> > 
> > 
> >>So as far as I can see there is *no way* to get 3 minutes worth of
> buffering
> >>by adjusting these parameters.
> >>
> >>Just to remind everyone - I have video data coming in at 25MBytes/sec and
> I
> >>am writing it out to a usb2 hard disk that can sustain 17MBytes/sec. I
> want
> >>my video capture to run at full speed as long as possible by having the
> >>7MBytes/sec deficit slowly eat up the available RAM in the machine. I
> have
> >>1.5Gb of RAM, 1.3Gb available for buffers, so this should take 3 minutes
> to
> >>consume at 7MBytes/sec.
> >>
> >>So, I've tried all the combinations on dirty_ratio and
> >>dirty_background_ratio and they *do not help*.
> >>
> > 
> > dirty_ratio is the tubable you want, if it's not working correctly,
> > either there's a problem with your setup, or a bug
> > 
> > 
> >>Can anyone suggest something else that I might try? The goal is to have
> >>25MBytes/sec streaming video for about 3 minutes. 
> >>
> > 
> > how sure are you that you're getting 17MB/s during this test?  can you
> > run "vmstat 1" while this is running to verify?  which FS and
> > scheduler?
> > 
> > just for interest, what's the raw disk bandwidth (use hdparm, or run a
> > dd, or something)?  it would obviously be much better to sustain
> > 25MB/s to disk
> > 
> > 
> >>Or is this simply not possible with the current kernel I/O setup? Do I
> have
> >>to do something elaborate myself, like build a big RAM buffer, mount the
> >>disk synchronous, do the buffering myself in userland...??
> >>
> > 
> > this should be possible, although it could be considered a bit risky WRT
> OOM.
> > 
> > NATE
> 
> -- 
> Anthony Wesley
> Director and IT/Network Consultant
> Smart Networks Pty Ltd
> Acquerra Pty Ltd
> 
> Anthony.Wesley@acquerra.com.au
> Phone: (02) 62595404 or 0419409836
>

Re: kernel 2.6.13 buffer strangeness

[Anthony Wesley]

[-- Attachment #1: Type: text/plain, Size: 5095 bytes --]

Nate, this test was done with the cfq scheduler, but I've tried them 
all, and they didn't seem to make any real difference. Let
me know if you want me to re-run this with one of the other schedulers. 
The default scheduler is anticipatory, and that's where
the problem first showed up.

Attached is the output of "vmstat -S K 1". I started the capture a 
couple of seconds after starting this trace and stopped it after 2 minutes.

If I'm interpreting it correctly, this shows a disk throughput averaging 
about 15-16MBytes/sec.

I saw the video write speed sustain 25Mbytes/sec for about 75 seconds 
and then it dropped.

dirty_background_ratio is set to 1
dirty_ratio is set to 95


Assuming that these numbers are ok, that indicates a deficit of about 
10MBytes/sec between my video in and disk out, so
for 1.3Gb of ram as buffer I should see about 130 seconds at full speed 
before it drops. Instead I see this happen after
about 75 seconds.

Thanks again,

Anthony

Nate Diller wrote:

>On 9/9/05, Anthony Wesley <awesley@acquerra.com.au> wrote:
>  
>
>>Okay, just tested a couple of things, here's what I see...
>>
>>I tested the write speed to the usb2 hard disk and got 21MBytes/sec. It's a
>>laptop 
>>hard drive, only 5400rpm so this is not surprising.
>>
>>I did this test with my video capture app running, but just displaying data
>>and not writing,
>>
>>I have another laptop usb2 hard drive here which I just tested and got
>>17MBytes/sec - it's
>>a 4200rpm drive so again not surprising numbers.
>>
>>The video data is written as individual frames, so the efficiency is a bit
>>below the raw throughput,
>>but my tests were transferring 1.5Gb of data as raw frames - exactly the
>>same way that Coriander would
>>write its data.
>>
>>I'd bet a large sum of money that these hard disk figures are correct to
>>within a few percent.
>>
>>Also, when I am actually capturing I have timed by stopwatch how long the
>>disk activity light
>>is on, and reached about the same conclusion.
>>
>>Working the problem from the other direction, the only way to explain the
>>early throttling that
>>I see would be if *almost no data* is being written to the disk, and this is
>>plainly not the case. Even if
>>the disk were running at a greatly reduced rate of (say) 10MBytes/sec I
>>would still see 86 seconds
>>of buffering before the throttle kicks in, and so far I have managed only to
>>get to about 65 or 70.
>>
>>regards, Anthony
>>
>>    
>>
>i need a vmstat trace ('vmstat 1 > logfile', then run the test, then
>^c)  before i can comment further.  also, your filesystem and
>scheduler would be interesting to know.  you can find out the
>scheduler with 'cat /sys/block/[drive]/queue/scheduler'.
>
>NATE
>
>  
>
>>Nate Diller wrote:
>>
>>
>>    
>>
>>>>Setting dirty_ratio and dirty_background_ratio to 90/10 puts me back at
>>>>around 50 seconds, i.e. where I started.
>>>>
>>>>        
>>>>
>>>this setting should do the trick, so there's something going on here
>>>that isn't expected.
>>>
>>>
>>>      
>>>
>>>>So as far as I can see there is *no way* to get 3 minutes worth of
>>>>        
>>>>
>>buffering
>>    
>>
>>>>by adjusting these parameters.
>>>>
>>>>Just to remind everyone - I have video data coming in at 25MBytes/sec and
>>>>        
>>>>
>>I
>>    
>>
>>>>am writing it out to a usb2 hard disk that can sustain 17MBytes/sec. I
>>>>        
>>>>
>>want
>>    
>>
>>>>my video capture to run at full speed as long as possible by having the
>>>>7MBytes/sec deficit slowly eat up the available RAM in the machine. I
>>>>        
>>>>
>>have
>>    
>>
>>>>1.5Gb of RAM, 1.3Gb available for buffers, so this should take 3 minutes
>>>>        
>>>>
>>to
>>    
>>
>>>>consume at 7MBytes/sec.
>>>>
>>>>So, I've tried all the combinations on dirty_ratio and
>>>>dirty_background_ratio and they *do not help*.
>>>>
>>>>        
>>>>
>>>dirty_ratio is the tubable you want, if it's not working correctly,
>>>either there's a problem with your setup, or a bug
>>>
>>>
>>>      
>>>
>>>>Can anyone suggest something else that I might try? The goal is to have
>>>>25MBytes/sec streaming video for about 3 minutes. 
>>>>
>>>>        
>>>>
>>>how sure are you that you're getting 17MB/s during this test?  can you
>>>run "vmstat 1" while this is running to verify?  which FS and
>>>scheduler?
>>>
>>>just for interest, what's the raw disk bandwidth (use hdparm, or run a
>>>dd, or something)?  it would obviously be much better to sustain
>>>25MB/s to disk
>>>
>>>
>>>      
>>>
>>>>Or is this simply not possible with the current kernel I/O setup? Do I
>>>>        
>>>>
>>have
>>    
>>
>>>>to do something elaborate myself, like build a big RAM buffer, mount the
>>>>disk synchronous, do the buffering myself in userland...??
>>>>
>>>>        
>>>>
>>>this should be possible, although it could be considered a bit risky WRT
>>>      
>>>
>>OOM.
>>    
>>
>>>NATE
>>>      
>>>
>>-- 
>>Anthony Wesley
>>Director and IT/Network Consultant
>>Smart Networks Pty Ltd
>>Acquerra Pty Ltd
>>
>>Anthony.Wesley@acquerra.com.au
>>Phone: (02) 62595404 or 0419409836
>>
>>    
>>
>
>  
>


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Re: kernel 2.6.13 buffer strangeness

[Anthony Wesley]

Nate Diller wrote:
> just found the culprit.  guess i should have read the code the first 
> time.  get_dirty_limits() in drivers/block/page_writeback.c has a 
> hard-coded upper limit to dirty_ratio.  it's capped to half of the 
> unmapped pages, so maybe 30-40% of your system's memory.  so if you are 
> brave, just remove the "/ 2" parts from the 'if (dirty_ratio > 
> unmapped_ratio / 2) dirty_ratio = unmapped_ratio / 2;' check, and you 
> can have all the OOM goodness you want.

Excellent. OOM here I come.

> i really recommend you focus on getting better disk bandwidth, you stand 
> to gain a lot more from that approach.  i presume you're on ext3; 
> perhaps you should try reiser4 or xfs, they are more likely to meet your 
> disk bandwidth requirements.

Yep, pursuing this as well, also looking to add more RAM to the machine. At this stage
I was just trying to understand the numbers that I was seeing, so I could work out the
best way to proceed.

I'm using ext2 at the moment, on the assumption that the journal would cost 
me a bit of performance so I left it out :-)

I'll certainly try the other filesystems as you suggest.

Thanks again, Anthony

-- 
Anthony Wesley
Director and IT/Network Consultant
Smart Networks Pty Ltd
Acquerra Pty Ltd

Anthony.Wesley@acquerra.com.au
Phone: (02) 62595404 or 0419409836

Re: kernel 2.6.13 buffer strangeness - FIXED

[Anthony Wesley]

Nate Diller wrote:

> just found the culprit.  guess i should have read the code the first 
> time.  get_dirty_limits() in drivers/block/page_writeback.c has a 
> hard-coded upper limit to dirty_ratio.  it's capped to half of the 
> unmapped pages, so maybe 30-40% of your system's memory.  so if you are 
> brave, just remove the "/ 2" parts from the 'if (dirty_ratio > 
> unmapped_ratio / 2) dirty_ratio = unmapped_ratio / 2;' check, and you 
> can have all the OOM goodness you want.

Well well well.

Thank you Nate!

I changed that bit of code to:

	if (dirty_ratio > unmapped_ratio - 10) 
		dirty_ratio = unmapped_ratio - 10;

and added a couple of sanity checks so that it couldn't get below 5 or above 95.

Then set /proc/sys/vm/dirty_ratio to 95 and dirty_background_ratio to 1.

Guess what? Now my video streams for 120 seconds before being throttled. This is *much better*.
It's now quite feasible to reach 30 seconds by finding a faster disk and/or adding another 512M
or RAM.

I guess the hard-coded limit is in there for a reason, but I sure wish it was adjustable without
this sort of hacking.

regards, Anthony

-- 
Anthony Wesley
Director and IT/Network Consultant
Smart Networks Pty Ltd
Acquerra Pty Ltd

Anthony.Wesley@acquerra.com.au
Phone: (02) 62595404 or 0419409836

Re: kernel 2.6.13 buffer strangeness - ext2/3/reiser4/xfs comparison

[Anthony Wesley]

Nate Diller wrote:
> i really recommend you focus on getting better disk bandwidth, you stand 
> to gain a lot more from that approach.  i presume you're on ext3; 
> perhaps you should try reiser4 or xfs, they are more likely to meet your 
> disk bandwidth requirements.
> 
> NATE

While I have already solved the issue that was troubling me, I also spent some time comparing different
filesystems as reccommended by Nate, with interesting results.

My method was simple - make a filesystem and the set it as the target for my video capture. With video
coming in at 25MBytes/sec and going out to disk at about 15-20MBytes/sec it is an interesting test of the
vm and filesystem.

I compared ext2,ext3,xfs,vfat,reiser and reiser4.

The hands-down winner was ext2. All the others showed problems of either lower disk throughput
or dropped frames during video capture.

Only ext2 went the full distance - no dropped frames until we run out of RAM, and good disk throughput.

xfs,reiser and reiser4 had slightly higher disk write speed, but showed performance problems
that caused lots of dropped frames so they must be ruled out at this stage.

I know that xfs and reiser4 are supposed to be faster for some things, but it seems to me that they
are not the best choice when you are predominantly writing lots and lots of 600k files :-)

regards, Anthony

-- 
Anthony Wesley
Director and IT/Network Consultant
Smart Networks Pty Ltd
Acquerra Pty Ltd

Anthony.Wesley@acquerra.com.au
Phone: (02) 62595404 or 0419409836

Re: kernel 2.6.13 buffer strangeness - ext2/3/reiser4/xfs comparison

[Andrew Morton]

Anthony Wesley <awesley@acquerra.com.au> wrote:
>
> I compared ext2,ext3,xfs,vfat,reiser and reiser4.
> 
>  The hands-down winner was ext2. All the others showed problems of either lower disk throughput
>  or dropped frames during video capture.

ext2 is a good filesystem.  For that sort of application all the journaling
gunk can really get in the way.

You should have tested ext3 with data=writeback.

Re: kernel 2.6.13 buffer strangeness - ext2/3/reiser4/xfs comparison

[Anthony Wesley]

Andrew Morton wrote:

> Anthony Wesley <awesley@acquerra.com.au> wrote:
> 
>>I compared ext2,ext3,xfs,vfat,reiser and reiser4.
>>
>> The hands-down winner was ext2. All the others showed problems of either lower disk throughput
>> or dropped frames during video capture.
> 
> 
> ext2 is a good filesystem.  For that sort of application all the journaling
> gunk can really get in the way.
> 
> You should have tested ext3 with data=writeback.
> 

Ask and ye shall receive...

I created an ext3 fs, mounted it with data=writeback and gave it a quick spin.

The result? Lots of pauses and dropped frames during capture. This is during the part of the
process where I have gobs of free RAM that's being used for buffering so dropping frames here
is a cardinal sin.

Dunno why it's happening, but I saw it also with xfs and reiser4. ext2 on the other hand
chugs along happily, no pauses, no dropped frames until we run out of free RAM (takes about 2
minutes now after the simple kernel change).

I can understand dropped frames after we run out of ram, but not before.

regards, Anthony

-- 
Anthony Wesley
Director and IT/Network Consultant
Smart Networks Pty Ltd
Acquerra Pty Ltd

Anthony.Wesley@acquerra.com.au
Phone: (02) 62595404 or 0419409836

Re: kernel 2.6.13 buffer strangeness

[Anthony Wesley]

Based on a better understanding of dirty_ratio and dirty_background ratio (thanks Nate) I just tried
the following test:

dirty_ratio set to 95
dirty_background_ratio set to 1

>From Nate's description of these parameters, this should mean that the disk writes
start almost immediately, and the kernel will allow 95% of RAM to become dirty before
applying the throttle.

Ok, so with 25Mbytes/s coming in, and 17Mbytes/sec going out to disk, the dirty pages should be growing
at 7Mbytes/sec. With these parameters set as above I should see about 3 minutes of full speed
video before the throttle is applied since I have about 1.3Gb of RAM free for buffering..

*But* when I try this experiment I hit the throttle after only 65 seconds - an improvement to 
be sure, but still a long way short of the 180 seconds that it ought to take.

Part of the test works as expected - the disk writes begin almost immediately due to the low
value for dirty_background_ratio, but the rest is a mystery.

It really looks as if the pages aren't being marked as clean fast enough after they're written.

How else can it take only 70 seconds to reach 95% dirty when I have 1.3Gb of available RAM and data coming in at 25MBytes/sec and out at 17MBytes/sec? It doesn't make any sense...

regards, Anthony

Nate Diller wrote:
> yes, on 2.6 there are two tunables which are important here. 
> dirty_background_ratio is the threshold where the kernel will begin
> flushing dirty buffers, so it should change how soon the disk becomes
> active.  dirty_ratio changes when the write-throttling code kicks in,
> which is what Anthony is seeing.  The purpose of the write throttling
> code is to limit the dirtying process to disk bandwidth, so that is a
> Feature.  Anthony, try *increasing* dirty_ratio, you can go up to 100,
> but you could trigger an OOM if you let it get too high, so maybe try
> setting it at 85 or so.  This should effectively disable the write
> throttling and give you the bandwidth you want.
> 
> NATE

-- 
Anthony Wesley
Director and IT/Network Consultant
Smart Networks Pty Ltd
Acquerra Pty Ltd

Anthony.Wesley@acquerra.com.au
Phone: (02) 62595404 or 0419409836

Re: kernel 2.6.13 buffer strangeness

[Andrew Morton]

Anthony Wesley <awesley@acquerra.com.au> wrote:
>
>  How else can it take only 70 seconds to reach 95% dirty when I have 1.3Gb of available RAM and data coming in at 25MBytes/sec and out at 17MBytes/sec? It doesn't make any sense...

What architecture?   x86?

If so then bear in mind that your memory is split into 500MB highmem and
800MB lowmem.  The kernel might be starting I/O due to the highmem zone
being full of dirty pages.  That'd be wrong of it if so - it's supposed to
just fall back to lowmem for the page allocations, but that code has
changed quite a bit in the two years since I got all that working...

You need to run `watch -n1 cat /proc/meminfo' while doing these tests...