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Date: Fri, 18 Dec 2009 22:03:05 +0100
From: Corrado Zoccolo <czoccolo@...il.com>
To: Jeff Moyer <jmoyer@...hat.com>
Cc: Linux-Kernel <linux-kernel@...r.kernel.org>,
Jens Axboe <jens.axboe@...cle.com>,
Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] cfq-iosched: reduce write depth only if sync was delayed
Thanks Jeff.
Your tests show that with the patch, low_latency doesn't penalize
writeback any more on fast hardware, so the goal of the patch is
fulfilled. Lowering slice_async doesn't change the picture (I was
expecting an improvement in seq write bandwidth).
I'm still puzzled of why with 2.6.29 your SAN could achieve 300 MB/s
sequential writes, and it can't achieve it any more.
Can you try lowering slice_idle, trying e.g. 2ms?
Thanks
Corrado
On Fri, Dec 18, 2009 at 4:32 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Corrado Zoccolo <czoccolo@...il.com> writes:
>
>> Hi Jeff,
>> On Wed, Dec 9, 2009 at 7:09 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
>>> Corrado Zoccolo <czoccolo@...il.com> writes:
>>>
>>>> The numbers look good. Now, there is no penalty in having low_latency
>>>> set for sequential writes, and just a small penalty for random ones.
>>>> The fact that random reads are faster with low_latency set is interesting.
>>>> Is the test is running with your patched tiobench (so that the number
>>>> of random operations is comparable with sequential ones)?
>>>
>>> No, I forgot all about that. The number of random operations defaults
>>> to 4000, which is pretty low. I'll re-run the tests with a number
>>> comparable to the sequential runs. Sorry about that.
>>>
>> N.P.
>> if you have time, can you also re-run the test changing:
>> iosched/fifo_expire_async to 8 ?
>> I hope that reducing the expire_async, will make cfq quicker at switching
>> between the different threads, allowing more parallelism for seq
>> writers on your hw.
>> If this is the case, I think I can try to estimate the
>> fifo_expire_async in the autotuning patch.
>
> Sorry this took so long. I've been rather busy of late.
>
> Cheers,
> Jeff
>
> low_latency=1, fifo_expire_async=8
>
> Unit information
> ================
> File size = megabytes
> Blk Size = bytes
> Rate = megabytes per second
> CPU% = percentage of CPU used during the test
> Latency = milliseconds
> Lat% = percent of requests that took longer than X seconds
> CPU Eff = Rate divided by CPU% - throughput per cpu load
>
> Sequential Reads
> File Blk Num Avg Maximum Lat% Lat% CPU
> Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
> ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
> 2.6.32 8192 65536 8 88.39 89.74% 16.388 2032.62 0.00000 0.00000 98
> 2.6.32 8192 65536 16 90.77 185.3% 32.213 2175.99 0.00000 0.00000 49
>
> Random Reads
> File Blk Num Avg Maximum Lat% Lat% CPU
> Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
> ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
> 2.6.32 8192 65536 8 19.62 25.74% 71.827 3397.26 0.00000 0.00000 76
> 2.6.32 8192 65536 16 23.82 55.01% 103.361 4075.53 0.00000 0.00000 43
>
> Sequential Writes
> File Blk Num Avg Maximum Lat% Lat% CPU
> Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
> ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
> 2.6.32 8192 65536 8 108.28 1007.% 12.984 5643.55 0.00076 0.00000 11
> 2.6.32 8192 65536 16 112.40 2014.% 25.430 8592.98 0.00839 0.00000 6
>
> Random Writes
> File Blk Num Avg Maximum Lat% Lat% CPU
> Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
> ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
> 2.6.32 8192 65536 8 63.94 337.7% 22.885 6047.22 0.00076 0.00000 19
> 2.6.32 8192 65536 16 61.94 662.5% 46.997 12759.69 0.15411 0.00000 9
>
>
> low_latency=0, fifo_expire_async=8
>
> Unit information
> ================
> File size = megabytes
> Blk Size = bytes
> Rate = megabytes per second
> CPU% = percentage of CPU used during the test
> Latency = milliseconds
> Lat% = percent of requests that took longer than X seconds
> CPU Eff = Rate divided by CPU% - throughput per cpu load
>
> Sequential Reads
> File Blk Num Avg Maximum Lat% Lat% CPU
> Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
> ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
> 2.6.32 8192 65536 8 89.13 88.30% 15.872 3101.39 0.00000 0.00000 101
> 2.6.32 8192 65536 16 86.78 161.7% 30.794 4909.02 0.00000 0.00000 54
>
> Random Reads
> File Blk Num Avg Maximum Lat% Lat% CPU
> Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
> ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
> 2.6.32 8192 65536 8 20.21 26.64% 69.863 4285.42 0.00000 0.00000 76
> 2.6.32 8192 65536 16 20.10 52.75% 139.761 5986.94 0.00076 0.00000 38
>
> Sequential Writes
> File Blk Num Avg Maximum Lat% Lat% CPU
> Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
> ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
> 2.6.32 8192 65536 8 108.74 1020.% 13.070 5331.78 0.00076 0.00000 11
> 2.6.32 8192 65536 16 112.18 2020.% 25.559 7903.16 0.00992 0.00000 6
>
> Random Writes
> File Blk Num Avg Maximum Lat% Lat% CPU
> Identifier Size Size Thr Rate (CPU%) Latency Latency >2s >10s Eff
> ---------------------------- ------ ----- --- ------ ------ --------- ----------- -------- -------- -----
> 2.6.32 8192 65536 8 64.53 337.8% 22.671 5388.77 0.00000 0.00000 19
> 2.6.32 8192 65536 16 61.75 668.9% 47.265 13271.37 0.12894 0.00000 9
>
--
__________________________________________________________________________
dott. Corrado Zoccolo mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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The self-confidence of a warrior is not the self-confidence of the average
man. The average man seeks certainty in the eyes of the onlooker and calls
that self-confidence. The warrior seeks impeccability in his own eyes and
calls that humbleness.
Tales of Power - C. Castaneda
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