Re: performance "regression" in cfq compared to anticipatory, deadline and noop

[Fabio Checconi]

Hi,

> From: Daniel J Blueman <daniel.blueman@xxxxxxxxx>
> Date: Sun, Aug 24, 2008 09:24:37PM +0100
>
> Hi Fabio, Jens,
> 
...
> This was the last test I didn't get around to. Alas, is did help, but
> didn't give the merging required for full performance:
> 
> # echo 1 >/proc/sys/vm/drop_caches; dd if=/dev/sda of=/dev/null
> bs=128k count=2000
> 262144000 bytes (262 MB) copied, 2.47787 s, 106 MB/s
> 
> # echo 1 >/proc/sys/vm/drop_caches; hdparm -t /dev/sda
> Timing buffered disk reads:  308 MB in  3.01 seconds = 102.46 MB/sec
> 
> It is an improvement over the baseline performance of 2.6.27-rc4:
> 
> # echo 1 >/proc/sys/vm/drop_caches; dd if=/dev/sda of=/dev/null
> bs=128k count=2000
> 262144000 bytes (262 MB) copied, 2.56514 s, 102 MB/s
> 
> # echo 1 >/proc/sys/vm/drop_caches; hdparm -t /dev/sda
> Timing buffered disk reads:  294 MB in  3.02 seconds =  97.33 MB/sec
> 
> Note that platter speed is around 125MB/s (which I get near at smaller
> read sizes).
> 
> I feel 128KB read requests are perhaps important, as this is a
> commonly-used RAID stripe size, and may explain the read-performance
> drop sometimes we see in hardware vs software RAID benchmarks.
> 
> How can we generate some ideas or movement on fixing/improving this behaviour?
> 

Thank you for testing.  The blktrace output for this run should be
interesting, esp. to compare it with a blktrace obtained from anticipatory
with the same workload - IIRC anticipatory didn't suffer from the problem,
and anticipatory has a slightly different dispatching mechanism that
this patch tried to bring into cfq.

Even if a proper fix may not belong to the elevator itself, I think
that this couple (this last test + anticipatory) of traces should help
in better understanding what is still going wrong.

Thank you in advance.
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