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SubjectRe: [Fwd: Re: [PATCH v2 2/2] x86: add prefetching to do_csum]
On Mon, Nov 11, 2013 at 05:42:22PM -0800, Joe Perches wrote:
> Hi again Neil.
>
> Forwarding on to netdev with a concern as to how often
> do_csum is used via csum_partial for very short headers
> and what impact any prefetch would have there.
>
> Also, what changed in your test environment?
>
> Why are the new values 5+% higher cycles/byte than the
> previous values?
>
Hmm, thank you, I didn't notice the increase. I think I rebooted my system and
failed to reset my irq affinity to avoid the processor I was testing on. Let me
rerun.
Neil

> And here is the new table reformatted:
>
> len set iterations Readahead cachelines vs cycles/byte
> 1 2 3 4 6 10 20
> 1500B 64MB 1000000 1.4342 1.4300 1.4350 1.4350 1.4396 1.4315 1.4555
> 1500B 128MB 1000000 1.4312 1.4346 1.4271 1.4284 1.4376 1.4318 1.4431
> 1500B 256MB 1000000 1.4309 1.4254 1.4316 1.4308 1.4418 1.4304 1.4367
> 1500B 512MB 1000000 1.4534 1.4516 1.4523 1.4563 1.4554 1.4644 1.4590
> 9000B 64MB 1000000 0.8921 0.8924 0.8932 0.8949 0.8952 0.8939 0.8985
> 9000B 128MB 1000000 0.8841 0.8856 0.8845 0.8854 0.8861 0.8879 0.8861
> 9000B 256MB 1000000 0.8806 0.8821 0.8813 0.8833 0.8814 0.8827 0.8895
> 9000B 512MB 1000000 0.8838 0.8852 0.8841 0.8865 0.8846 0.8901 0.8865
> 64KB 64MB 1000000 0.8132 0.8136 0.8132 0.8150 0.8147 0.8149 0.8147
> 64KB 128MB 1000000 0.8013 0.8014 0.8013 0.8020 0.8041 0.8015 0.8033
> 64KB 256MB 1000000 0.7956 0.7959 0.7956 0.7976 0.7981 0.7967 0.7973
> 64KB 512MB 1000000 0.7934 0.7932 0.7937 0.7951 0.7954 0.7943 0.7948
>
> -------- Forwarded Message --------
> From: Neil Horman <nhorman@tuxdriver.com>
> To: Joe Perches <joe@perches.com>
> Cc: Dave Jones <davej@redhat.com>, linux-kernel@vger.kernel.org,
> sebastien.dugue@bull.net, Thomas Gleixner <tglx@linutronix.de>, Ingo
> Molnar <mingo@redhat.com>, H. Peter Anvin <hpa@zytor.com>,
> x86@kernel.org
> Subject: Re: [PATCH v2 2/2] x86: add prefetching to do_csum
>
> On Fri, Nov 08, 2013 at 12:29:07PM -0800, Joe Perches wrote:
> > On Fri, 2013-11-08 at 15:14 -0500, Neil Horman wrote:
> > > On Fri, Nov 08, 2013 at 11:33:13AM -0800, Joe Perches wrote:
> > > > On Fri, 2013-11-08 at 14:01 -0500, Neil Horman wrote:
> > > > > On Wed, Nov 06, 2013 at 09:19:23AM -0800, Joe Perches wrote:
> > > > > > On Wed, 2013-11-06 at 10:54 -0500, Neil Horman wrote:
> > > > > > > On Wed, Nov 06, 2013 at 10:34:29AM -0500, Dave Jones wrote:
> > > > > > > > On Wed, Nov 06, 2013 at 10:23:19AM -0500, Neil Horman wrote:
> > > > > > > > > do_csum was identified via perf recently as a hot spot when doing
> > > > > > > > > receive on ip over infiniband workloads. After alot of testing and
> > > > > > > > > ideas, we found the best optimization available to us currently is to
> > > > > > > > > prefetch the entire data buffer prior to doing the checksum
> > > > > > []
> > > > > > > I'll fix this up and send a v3, but I'll give it a day in case there are more
> > > > > > > comments first.
> > > > > >
> > > > > > Perhaps a reduction in prefetch loop count helps.
> > > > > >
> > > > > > Was capping the amount prefetched and letting the
> > > > > > hardware prefetch also tested?
> > > > > >
> > > > > > prefetch_lines(buff, min(len, cache_line_size() * 8u));
> > > > > >
> > > > >
> > > > > Just tested this out:
> > > >
> > > > Thanks.
> > > >
> > > > Reformatting the table so it's a bit more
> > > > readable/comparable for me:
> > > >
> > > > len SetSz Loops cycles/byte
> > > > limited unlimited
> > > > 1500B 64MB 1M 1.3442 1.3605
> > > > 1500B 128MB 1M 1.3410 1.3542
> > > > 1500B 256MB 1M 1.3536 1.3710
> > > > 1500B 512MB 1M 1.3463 1.3536
> > > > 9000B 64MB 1M 0.8522 0.8504
> > > > 9000B 128MB 1M 0.8528 0.8536
> > > > 9000B 256MB 1M 0.8532 0.8520
> > > > 9000B 512MB 1M 0.8527 0.8525
> > > > 64KB 64MB 1M 0.7686 0.7683
> > > > 64KB 128MB 1M 0.7695 0.7686
> > > > 64KB 256MB 1M 0.7699 0.7708
> > > > 64KB 512MB 1M 0.7799 0.7694
> > > >
> > > > This data appears to show some value
> > > > in capping for 1500b lengths and noise
> > > > for shorter and longer lengths.
> > > >
> > > > Any idea what the actual distribution of
> > > > do_csum lengths is under various loads?
> > > >
> > > I don't have any hard data no, sorry.
> >
> > I think you should before you implement this.
> > You might find extremely short lengths.
> >
> > > I'll cap the prefetch at 1500B for now, since it
> > > doesn't seem to hurt or help beyond that
> >
> > The table data has a max prefetch of
> > 8 * boot_cpu_data.x86_cache_alignment so
> > I believe it's always less than 1500 but
> > perhaps 4 might be slightly better still.
> >
>
>
> So, you appear to be correct, I reran my test set with different prefetch
> ceilings and got the results below. There are some cases in which there is a
> performance gain, but the gain is small, and occurs at different spots depending
> on the input buffer size (though most peak gains appear around 2 cache lines).
> I'm guessing it takes about 2 prefetches before hardware prefetching catches up,
> at which point we're just spending time issuing instructions that get discarded.
> Given the small prefetch limit, and the limited gains (which may also change on
> different hardware), I think we should probably just drop the prefetch idea
> entirely, and perhaps just take the perf patch so that we can revisit this area
> when hardware that supports the avx extensions and/or adcx/adox becomes
> available.
>
> Ingo, does that seem reasonable to you?
> Neil
>
>
>
> 1 cache line:
> len | set | iterations | cycles/byte
> ========|=======|===============|=============
> 1500B | 64MB | 1000000 | 1.434190
> 1500B | 128MB | 1000000 | 1.431216
> 1500B | 256MB | 1000000 | 1.430888
> 1500B | 512MB | 1000000 | 1.453422
> 9000B | 64MB | 1000000 | 0.892055
> 9000B | 128MB | 1000000 | 0.884050
> 9000B | 256MB | 1000000 | 0.880551
> 9000B | 512MB | 1000000 | 0.883848
> 64KB | 64MB | 1000000 | 0.813187
> 64KB | 128MB | 1000000 | 0.801326
> 64KB | 256MB | 1000000 | 0.795643
> 64KB | 512MB | 1000000 | 0.793400
>
>
> 2 cache lines:
> len | set | iterations | cycles/byte
> ========|=======|===============|=============
> 1500B | 64MB | 1000000 | 1.430030
> 1500B | 128MB | 1000000 | 1.434589
> 1500B | 256MB | 1000000 | 1.425430
> 1500B | 512MB | 1000000 | 1.451570
> 9000B | 64MB | 1000000 | 0.892369
> 9000B | 128MB | 1000000 | 0.885577
> 9000B | 256MB | 1000000 | 0.882091
> 9000B | 512MB | 1000000 | 0.885201
> 64KB | 64MB | 1000000 | 0.813629
> 64KB | 128MB | 1000000 | 0.801377
> 64KB | 256MB | 1000000 | 0.795861
> 64KB | 512MB | 1000000 | 0.793242
>
> 3 cache lines:
> len | set | iterations | cycles/byte
> ========|=======|===============|=============
> 1500B | 64MB | 1000000 | 1.435048
> 1500B | 128MB | 1000000 | 1.427103
> 1500B | 256MB | 1000000 | 1.431558
> 1500B | 512MB | 1000000 | 1.452250
> 9000B | 64MB | 1000000 | 0.893162
> 9000B | 128MB | 1000000 | 0.884488
> 9000B | 256MB | 1000000 | 0.881314
> 9000B | 512MB | 1000000 | 0.884060
> 64KB | 64MB | 1000000 | 0.813185
> 64KB | 128MB | 1000000 | 0.801280
> 64KB | 256MB | 1000000 | 0.795554
> 64KB | 512MB | 1000000 | 0.793670
>
> 4 cache lines:
> len | set | iterations | cycles/byte
> ========|=======|===============|=============
> 1500B | 64MB | 1000000 | 1.435013
> 1500B | 128MB | 1000000 | 1.428434
> 1500B | 256MB | 1000000 | 1.430780
> 1500B | 512MB | 1000000 | 1.456285
> 9000B | 64MB | 1000000 | 0.894877
> 9000B | 128MB | 1000000 | 0.885387
> 9000B | 256MB | 1000000 | 0.883293
> 9000B | 512MB | 1000000 | 0.886462
> 64KB | 64MB | 1000000 | 0.815036
> 64KB | 128MB | 1000000 | 0.801962
> 64KB | 256MB | 1000000 | 0.797618
> 64KB | 512MB | 1000000 | 0.795138
>
> 6 cache lines:
> len | set | iterations | cycles/byte
> ========|=======|===============|=============
> 1500B | 64MB | 1000000 | 1.439609
> 1500B | 128MB | 1000000 | 1.437569
> 1500B | 256MB | 1000000 | 1.441776
> 1500B | 512MB | 1000000 | 1.455362
> 9000B | 64MB | 1000000 | 0.895242
> 9000B | 128MB | 1000000 | 0.886149
> 9000B | 256MB | 1000000 | 0.881375
> 9000B | 512MB | 1000000 | 0.884610
> 64KB | 64MB | 1000000 | 0.814658
> 64KB | 128MB | 1000000 | 0.804124
> 64KB | 256MB | 1000000 | 0.798143
> 64KB | 512MB | 1000000 | 0.795377
>
> 10 cache lines:
> len | set | iterations | cycles/byte
> ========|=======|===============|=============
> 1500B | 64MB | 1000000 | 1.431512
> 1500B | 128MB | 1000000 | 1.431805
> 1500B | 256MB | 1000000 | 1.430388
> 1500B | 512MB | 1000000 | 1.464370
> 9000B | 64MB | 1000000 | 0.893922
> 9000B | 128MB | 1000000 | 0.887852
> 9000B | 256MB | 1000000 | 0.882711
> 9000B | 512MB | 1000000 | 0.890067
> 64KB | 64MB | 1000000 | 0.814890
> 64KB | 128MB | 1000000 | 0.801470
> 64KB | 256MB | 1000000 | 0.796658
> 64KB | 512MB | 1000000 | 0.794266
>
> 20 cache lines:
> len | set | iterations | cycles/byte
> ========|=======|===============|=============
> 1500B | 64MB | 1000000 | 1.455539
> 1500B | 128MB | 1000000 | 1.443117
> 1500B | 256MB | 1000000 | 1.436739
> 1500B | 512MB | 1000000 | 1.458973
> 9000B | 64MB | 1000000 | 0.898470
> 9000B | 128MB | 1000000 | 0.886110
> 9000B | 256MB | 1000000 | 0.889549
> 9000B | 512MB | 1000000 | 0.886547
> 64KB | 64MB | 1000000 | 0.814665
> 64KB | 128MB | 1000000 | 0.803252
> 64KB | 256MB | 1000000 | 0.797268
> 64KB | 512MB | 1000000 | 0.794830
>
>
>
>


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