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Date: Mon, 11 Nov 2013 17:42:22 -0800 From: Joe Perches <joe@...ches.com> To: netdev <netdev@...r.kernel.org>, Neil Horman <nhorman@...driver.com> Cc: Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org, sebastien.dugue@...l.net, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, Eric Dumazet <eric.dumazet@...il.com> Subject: [Fwd: Re: [PATCH v2 2/2] x86: add prefetching to do_csum] 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? 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@...driver.com> To: Joe Perches <joe@...ches.com> Cc: Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org, sebastien.dugue@...l.net, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, H. Peter Anvin <hpa@...or.com>, x86@...nel.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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