Re: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest]

[Hans Reiser <reiser@namesys.com>]

Andrea Arcangeli wrote:

>On Mon, Feb 10, 2003 at 03:58:13PM +0300, Hans Reiser wrote:
>  
>
>>Is the following a fair summary?
>>
>>There is a certain minimum size required for the IOs to be cost 
>>effective.  This can be determined by single reader benchmarking.  Only 
>>readahead and not anticipatory scheduling addresses that.
>>
>>Anticipatory scheduling does not address the application that spends one 
>>minute processing every read that it makes.  Readahead does.
>>
>>Anticipatory scheduling does address the application that reads multiple 
>>files that are near each other (because they are in the same directory), 
>>and current readahead implementations (excepting reiser4 in progress 
>>vaporware) do not.
>>
>>Anticipatory scheduling can do a better job of avoiding unnecessary 
>>reads for workloads with small time gaps between reads than readahead 
>>(it is potentially more accurate for some workloads).
>>
>>Is this a fair summary?
>>    
>>
>
>I would also add what I feel the most important thing, that is
>anticipatory scheduling can help decreasing a lot the latencies of
>filesystem reads in presence of lots of misc I/O, by knowing which are
>the read commands that are intermediate-dependent-sync, that means
>knowing a new dependent read will be submitted very quickly as soon as
>the current read-I/O is completed.
>
Ah.... yes...  I have seen that be a problem, and reiserfs suffers from 
it more than ext2....

maybe we should simply have an explicit protocol for that...  I would be 
quite willing to have reiserfs use some flag that says 
WILL_READ_NEARBY_NEXT whenever it reads an internal node to get the 
location of its children....  that might be a lot cleaner than trying to 
do it all in your layer...  what do you guys think...

Perhaps it will be both less code, and a better result in that it avoids 
some unnecessary lingering....

> In such a case it makes lots sense to
>wait for the next read to be submitted instead of start processing
>immediatly the rest of the I/O queue.  This way when you read a file and
>you've to walk the indirect blocks before being able to read the data,
>you won't be greatly peanalized against the writes or reads that won't
>need to pass through metadata I/O before being served.
>
>This doesn't obviate the need of SFQ for the patological multimedia
>cases where I/O latency is the first prio, or for workloads where
>sync-write latency is the first prio.
>
>BTW, I'm also thinking that the SFQ could be selected not system wide,
>but per-process basis, with a prctl or something, so you could have all
>the I/O going into the single default async-io queue, except for mplayer
>that will use the SFQ queues. This may not even need to be privilegied
>since SFQ is fair and if an user can write a lot it can just hurt
>latency, with SFQ could hurt more but still not deadlock. This SFQ prctl
>for the I/O scheduler, would be very similar to the RT policy for the
>main process scheduler. Infact it maybe the best to just have SFQ always
>enabled, and selectable only via the SFQ prctl, and to enable the
>functionaltiy only per-process basis rather than global. We can still
>add a sysctl to enable it globally despite nobody set the per-process
>flag.
>
>Andrea
>
>
>  
>
I still don't really understand your SFQ design, probably because I 
haven't studied recent networking algorithms that your description 
assumed I understand.

-- 
Hans