how to turn down cpu usage of raid ?

how to turn down cpu usage of raid ?

[Matthew Simpson]

Help!  I am having complaints from users about CPU spikes when writing to my
RAID 1 array.  Is there a way I can tune software RAID so that writing
updates doesn't interfere with other applications?  [can I nice the raid1d
process?]

Specifically I experience giant slowdowns in a hosted application whenever
someone unpacks a tar file or the like.  Load averages are 1.10 - 2.00
during writes, but 0.00 to 0.05 otherwise.

root@ns2:~# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5]
read_ahead 1024 sectors
md0 : active raid1 hdc1[1] hda1[0]
      78979200 blocks [2/2] [UU]

unused devices: <none>

root@ns2:~# cat /etc/raidtab
raiddev /dev/md0
        raid-level      1
        nr-raid-disks   2
        nr-spare-disks  0
        chunk-size      4
        persistent-superblock   1
        device  /dev/hda1
        raid-disk       0
        device  /dev/hdc1
        raid-disk       1

This is a pentium IV 2.5GHz system with 1GB of RAM.
    IDE interface: PCI device 8086:24cb (Intel Corp.) (rev 1). -- 82820
Camino 2 chipset

Re: how to turn down cpu usage of raid ?

[Mark Hahn]

> Help!  I am having complaints from users about CPU spikes when writing to my
> RAID 1 array.

I can think of two answers: first, are you sure your drives are configured
sanely?  that is, using dma?  with any reasonable kernel, they should be,
but its possible to compile in the wrong driver or make some other mistake.
hdparm -iv /dev/hda and hdc should show using_dma=1.  you can also look
at /proc/ide/hda/settings.

second, perhaps you should simply make the kernel less lazy at starting
writes.  here's some basic settings from 2.4:

[hahn@hahn hahn]$ cat /proc/sys/vm/bdflush 
30      500     0       0       500     3000    60      20      0

 Value      Meaning                                                            
 nfract     Percentage of buffer cache dirty to activate bdflush              
 ndirty     Maximum number of dirty blocks to  write out per wake-cycle        
 dummy      Unused                                                             
 dummy      Unused                                                             
 interval   jiffies delay between kupdate flushes
 age_buffer Time for normal buffer to age before we flush it                   
 nfract_sync Percentage of buffer cache dirty to activate bdflush
 synchronously
 nfract_stop_bdflush Percetange of buffer cache dirty to stop bdflush
 dummy      Unused                                                


in theory, this means:
	- wake up bdflush when 30% of buffers are dirty.
	- write up to 500 blocks per wakeup.
	- 5 seconds between wakeups.
	- let a buffer age for 30 seconds before flushing it.
	- if 60% of buffers are dirty, start throttling dirtiers.
	- stop bdflush when < 20% of buffers are dirty.

of course, the code doesn't exactly do this, and 2.6 is very different.
still, I'm guessing that:
	- 500 buffers (pages, right?) is too little
	- 5 seconds is to infrequent
	- 30 seconds is probably too long

I have the fileserver for one of my clusters running much smoother with
ndirty=1000, interval=200 and age_buffer=1000.  my logic is that the disk
system can sustain around 200 MB/s, so flushing 4MB per wakeup is pretty 
minimal.  I also hate to see the typical burstiness of bdflush - no IO
between bursts at 5 second intervals.  I'd rather see a smoother stream of 
write-outs - perhaps even a 1-second interval.  finally, Unix's traditional
30-second laziness is mainly done in the hopes that a temporary file will be 
deleted before ever hitting the disk (and/or writes will be combined).  I 
think 30 seconds is an eternity nowadays, and 10 seconds is more reasonable.

in short:
echo '30 1000 0 0 200 1000 60 20 0' > /proc/sys/vm/bdflush

perhaps:
echo '30 1000 0 0 100 1000 60 20 0' > /proc/sys/vm/bdflush

for extra credit, investigate whether nfract=30 is too high (I think so, on
today's big-memory systems).  whether higher ndirty improves balance (these
writes would compete with application IO, so might hurt, albeit less with 
2.6's smarter IO scheduler.)  whether the sync/stop parameters make a
difference, too - throttling dirtiers should probably kick in earlier,
but if you lower nfract, also lower nfract_stop_bdflush...

> Is there a way I can tune software RAID so that writing
> updates doesn't interfere with other applications? 

remember also that many servers don't need atime updates; this can make a big 
difference in some cases.

Re: how to turn down cpu usage of raid ?

[Matthew Simpson]

No, I'm not using DMA.  :(

/dev/hda:
 multcount    =  0 (off)
 IO_support   =  0 (default 16-bit)
 unmaskirq    =  0 (off)
 using_dma    =  0 (off)
 keepsettings =  0 (off)
 readonly     =  0 (off)
 readahead    =  8 (on)
 geometry     = 10587/240/63, sectors = 160086528, start = 0

Model=Maxtor 6Y080P0, FwRev=YAR41BW0, SerialNo=Y24BG1QE
 Config={ Fixed }
 RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=57
 BuffType=DualPortCache, BuffSize=7936kB, MaxMultSect=16, MultSect=off
 CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=160086528
 IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
 PIO modes:  pio0 pio1 pio2 pio3 pio4
 DMA modes:  mdma0 mdma1 mdma2
 UDMA modes: udma0 udma1 udma2 udma3 udma4 *udma5 udma6
 AdvancedPM=yes: disabled (255) WriteCache=enabled
 Drive conforms to: :  1 2 3 4 5 6 7

hdc is the same of course, they are matched disks.

I did some googling on hdparm before and found out how to change things, but
I am nervous about changing my production servers after testing hdparm on
another server.  The problem is that my two production servers are using the
Intel chipset [same board], and the test server is using a Via chipset.  I
was able to set multcount to 16, IO support to 3 [32-bit sync], but I
tried -X66 -u1 -d1 and OOPSed the kernel.  Not sure if it was the -X66,
the -u1, or the -d1 that killed it, but I'm not sure what is safe to screw
with, and I can't hose a production server.  Of course the Intel boards
might be better....

Here is the other server, same Intel controller, but WDC disks instead of
Maxtor:

/dev/hda:
 multcount    =  0 (off)
 IO_support   =  0 (default 16-bit)
 unmaskirq    =  0 (off)
 using_dma    =  0 (off)
 keepsettings =  0 (off)
 readonly     =  0 (off)
 readahead    =  8 (on)
 geometry     = 9729/255/63, sectors = 156301488, start = 0

 Model=WDC WD800JB-00ETA0, FwRev=77.07W77, SerialNo=WD-WCAHL4821776
 Config={ HardSect NotMFM HdSw>15uSec SpinMotCtl Fixed DTR>5Mbs FmtGapReq }
 RawCHS=16383/16/63, TrkSize=57600, SectSize=600, ECCbytes=74
 BuffType=DualPortCache, BuffSize=8192kB, MaxMultSect=16, MultSect=off
 CurCHS=65535/1/63, CurSects=4128705, LBA=yes, LBAsects=156301488
 IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
 PIO modes:  pio0 pio1 pio2 pio3 pio4
 DMA modes:  mdma0 mdma1 mdma2
 UDMA modes: udma0 udma1 udma2 udma3 udma4 *udma5
 AdvancedPM=no WriteCache=enabled
 Drive conforms to: device does not report version:  1 2 3 4 5 6

What is the most important value to help this problem out?  I did some tests
with hdparm -Tt on the Via server, and adding multcount 16 and changing the
IO_support to 32-bit sync actually HURT performance instead of helping it.
If DMA is the biggest issue here, I can try turning turning that on and hope
for the best...

yours,
Matthew

----- Original Message ----- 
From: "Mark Hahn" <hahn@physics.mcmaster.ca>
To: "Matthew Simpson" <matthew@symatec-computer.com>
Cc: <linux-raid@vger.kernel.org>
Sent: Monday, February 02, 2004 7:37 PM
Subject: Re: how to turn down cpu usage of raid ?


> > Help!  I am having complaints from users about CPU spikes when writing
to my
> > RAID 1 array.
>
> I can think of two answers: first, are you sure your drives are configured
> sanely?  that is, using dma?  with any reasonable kernel, they should be,
> but its possible to compile in the wrong driver or make some other
mistake.
> hdparm -iv /dev/hda and hdc should show using_dma=1.  you can also look
> at /proc/ide/hda/settings.
>
> second, perhaps you should simply make the kernel less lazy at starting
> writes.  here's some basic settings from 2.4:
>
> [hahn@hahn hahn]$ cat /proc/sys/vm/bdflush
> 30      500     0       0       500     3000    60      20      0
>
>  Value      Meaning
>  nfract     Percentage of buffer cache dirty to activate bdflush
>  ndirty     Maximum number of dirty blocks to  write out per wake-cycle
>  dummy      Unused
>  dummy      Unused
>  interval   jiffies delay between kupdate flushes
>  age_buffer Time for normal buffer to age before we flush it
>  nfract_sync Percentage of buffer cache dirty to activate bdflush
>  synchronously
>  nfract_stop_bdflush Percetange of buffer cache dirty to stop bdflush
>  dummy      Unused
>
>
> in theory, this means:
> - wake up bdflush when 30% of buffers are dirty.
> - write up to 500 blocks per wakeup.
> - 5 seconds between wakeups.
> - let a buffer age for 30 seconds before flushing it.
> - if 60% of buffers are dirty, start throttling dirtiers.
> - stop bdflush when < 20% of buffers are dirty.
>
> of course, the code doesn't exactly do this, and 2.6 is very different.
> still, I'm guessing that:
> - 500 buffers (pages, right?) is too little
> - 5 seconds is to infrequent
> - 30 seconds is probably too long
>
> I have the fileserver for one of my clusters running much smoother with
> ndirty=1000, interval=200 and age_buffer=1000.  my logic is that the disk
> system can sustain around 200 MB/s, so flushing 4MB per wakeup is pretty
> minimal.  I also hate to see the typical burstiness of bdflush - no IO
> between bursts at 5 second intervals.  I'd rather see a smoother stream of
> write-outs - perhaps even a 1-second interval.  finally, Unix's
traditional
> 30-second laziness is mainly done in the hopes that a temporary file will
be
> deleted before ever hitting the disk (and/or writes will be combined).  I
> think 30 seconds is an eternity nowadays, and 10 seconds is more
reasonable.
>
> in short:
> echo '30 1000 0 0 200 1000 60 20 0' > /proc/sys/vm/bdflush
>
> perhaps:
> echo '30 1000 0 0 100 1000 60 20 0' > /proc/sys/vm/bdflush
>
> for extra credit, investigate whether nfract=30 is too high (I think so,
on
> today's big-memory systems).  whether higher ndirty improves balance
(these
> writes would compete with application IO, so might hurt, albeit less with
> 2.6's smarter IO scheduler.)  whether the sync/stop parameters make a
> difference, too - throttling dirtiers should probably kick in earlier,
> but if you lower nfract, also lower nfract_stop_bdflush...
>
> > Is there a way I can tune software RAID so that writing
> > updates doesn't interfere with other applications?
>
> remember also that many servers don't need atime updates; this can make a
big
> difference in some cases.
>
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

RE: how to turn down cpu usage of raid ?

[Guy]

Load and CPU usage are 2 different things, but somehow related.  What is the
CPU load during this time?

From what I have seen, the RAID software md, whatever, uses almost no CPU.
So, I don't think it is a CPU issue.  Even with RAID5, the CPU load is very
low.  Even during a rebuild!

Use top to see cpu usage.
Use sar (man sar) for keeping a log of cpu load and a lot of other stuff.

"sar -d" to see disk usage.

Maybe you are swapping, this would be bad.  Add RAM if so.

I bet your 2 disks are the bottle neck.

If the RAID1 can be tuned, I don't know how.  But the first step is to
determine what the bottle neck is.

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Matthew Simpson
Sent: Monday, February 02, 2004 6:18 PM
To: linux-raid@vger.kernel.org
Subject: how to turn down cpu usage of raid ?

Help!  I am having complaints from users about CPU spikes when writing to my
RAID 1 array.  Is there a way I can tune software RAID so that writing
updates doesn't interfere with other applications?  [can I nice the raid1d
process?]

Specifically I experience giant slowdowns in a hosted application whenever
someone unpacks a tar file or the like.  Load averages are 1.10 - 2.00
during writes, but 0.00 to 0.05 otherwise.

root@ns2:~# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5]
read_ahead 1024 sectors
md0 : active raid1 hdc1[1] hda1[0]
      78979200 blocks [2/2] [UU]

unused devices: <none>

root@ns2:~# cat /etc/raidtab
raiddev /dev/md0
        raid-level      1
        nr-raid-disks   2
        nr-spare-disks  0
        chunk-size      4
        persistent-superblock   1
        device  /dev/hda1
        raid-disk       0
        device  /dev/hdc1
        raid-disk       1

This is a pentium IV 2.5GHz system with 1GB of RAM.
    IDE interface: PCI device 8086:24cb (Intel Corp.) (rev 1). -- 82820
Camino 2 chipset


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