From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752785AbbCNJVO (ORCPT ); Sat, 14 Mar 2015 05:21:14 -0400 Received: from mail-we0-f178.google.com ([74.125.82.178]:32784 "EHLO mail-we0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751358AbbCNJVG (ORCPT ); Sat, 14 Mar 2015 05:21:06 -0400 Date: Sat, 14 Mar 2015 12:21:05 +0300 From: Alexey Dobriyan To: Rasmus Villemoes Cc: Linux Kernel , Andrew Morton , Peter Zijlstra , Tejun Heo , Denis Vlasenko , KAMEZAWA Hiroyuki Subject: Re: + lib-vsprintfc-even-faster-decimal-conversion.patch added to -mm tree Message-ID: <20150314092104.GA1674@p183.telecom.by> References: <5500b987.kerYYCYfIffruy3Z%akpm@linux-foundation.org> <87y4n0xxn3.fsf@rasmusvillemoes.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87y4n0xxn3.fsf@rasmusvillemoes.dk> User-Agent: Mutt/1.5.22 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Mar 14, 2015 at 12:53:20AM +0100, Rasmus Villemoes wrote: > On Fri, Mar 13 2015, Alexey Dobriyan wrote: > > > On Thu, Mar 12, 2015 at 12:54 AM, wrote: > >> Subject: lib/vsprintf.c: even faster binary to decimal conversion > > > > I spent some time to microbenchmark changes in userspace (audience: fool!). > > Results are below. > > > > Legend is "number avg+-1sigma min-max". Every number is CPU cycles. > > Great care was taken to remove interrupt noise. > > > > Number of measurements is 100 millions per line. > > CPU is Intel Core 2 Duo E6550 in 64-bit mode. > > > > 3.19.1: > > > > 0 98.015369 +- 0.512937 91-616 > > 42 116.000193 +- 3.523826 112-868 > > 27182 137.009008 +- 3.515725 133-1043 > > 65535 137.008262 +- 3.521761 133-840 > > 4294967295 201.019966 +- 3.278608 196-1050 > > 3141592653589793238 289.996882 +- 3.489376 287-1148 > > 18446744073709551615 295.065274 +- 2.860187 287-1029 > > ----------------------------------------------------- > > 3.19.1+patch > > 0 94.444063 +- 3.518922 84-630 > > 42 116.428533 +- 18.539093 105-1036 > > 42 116.316904 +- 18.234484 105-833 > > 27182 136.172398 +- 3.737113 133-980 > > 65535 136.014742 +- 3.537882 133-714 > > 4294967295 172.009618 +- 3.507473 168-826 > > 3141592653589793238 207.001114 +- 3.492724 196-1120 > > 18446744073709551615 208.018154 +- 3.220185 203-1246 > > ----------------------------------------------------- > > This seems to measure lfence+rdtsc overhead more than anything else. On > my presumably rather similar Core2 Duo T5870, I get an average of 22 > cycles for the old code and 11 cycles for the new when converting 42 two > million times in a loop, and I'm not even trying to take interrupts into > account. > > Since you seem to be dirtying 800 MB of memory, I'm guessing you get > quite a few page faults/TLB misses, which might explain the rather huge > max numbers. Huge max numbers come from RDTSC itself. If you measure tiniest possible RDTSC;RDTSC loop, it will reliably show huge results once in a while, on that box it is in 70-500 range IIRC. > > New code is somewhat faster for huge numbers. > > But top and ps don't show huge numbers normally -- > > it is either PIDs (2^16) or moderately high numbers in a range of millions > > (see /proc/stat) > > I said much the same thing in the commit log, and accordingly I've done > (micro)benchmarks with distributions biased to various degrees towards > smaller numbers, all of which showed 25+% improvement. > > > * variance for new code is bigger > > Seems to depend on how you measure... > > > I even tried N=42 twice because I thought 18.5 variance is a glitch > > but it is not. > > That does seem odd. But I think your numbers are caused by the huge > memory use. In any case, I modified my test program to record the > cycle count for each individual call (using lfence+rdtsc), but I used a > frequency table instead of a gigantic array, ignoring excessively large > cycle counts (I used > 1023). On the Core 2, I then get > > 90 90 90 80 80 80 80 80 80 80 80 80 90 80 80 > Distribution Function cycles/conv std.dev. (ignored) > uniform([10, 2^64-1]) linux_put_dec 224.83 9.80 (156) > uniform([10, 2^64-1]) rv_put_dec 147.87 7.44 (86) > 3 + neg_binom(0.05) linux_put_dec 138.97 39.48 (87) > 3 + neg_binom(0.05) rv_put_dec 123.76 27.33 (77) > 3 + neg_binom(0.10) linux_put_dec 115.49 27.27 (84) > 3 + neg_binom(0.10) rv_put_dec 108.22 20.14 (71) > 3 + neg_binom(0.15) linux_put_dec 105.21 20.52 (59) > 3 + neg_binom(0.15) rv_put_dec 101.75 17.21 (54) > 3 + neg_binom(0.20) linux_put_dec 100.79 17.25 (65) > 3 + neg_binom(0.20) rv_put_dec 98.34 16.22 (64) > 3 + neg_binom(0.50) linux_put_dec 87.84 7.75 (44) > 3 + neg_binom(0.50) rv_put_dec 85.37 8.26 (45) > > [first line is just deltas between a few lfence+rdtsc reads in quick > succession, to get a sense of the overhead]. For each distribution I'm > generating 2048 random numbers and then iterate over that 1000 times. So > almost none of the ~2M observations are being ignored. Here, the new > code is always faster (but computing a percentage from numbers including > the rdtsc overhead is meaningless), and in all but the last case (where > the numbers are almost exclusively 2-digit) the std. deviation is also > smaller. An an Intel Xeon, I get > > 48 32 32 32 32 32 32 32 32 32 32 32 32 32 32 > Distribution Function cycles/conv std.dev. (ignored) > uniform([10, 2^64-1]) linux_put_dec 152.58 8.54 (26) > uniform([10, 2^64-1]) rv_put_dec 89.33 3.02 (16) > 3 + neg_binom(0.05) linux_put_dec 91.88 34.46 (17) > 3 + neg_binom(0.05) rv_put_dec 71.59 21.19 (10) > 3 + neg_binom(0.10) linux_put_dec 72.50 25.30 (12) > 3 + neg_binom(0.10) rv_put_dec 60.10 17.51 (11) > 3 + neg_binom(0.15) linux_put_dec 63.81 20.68 (8) > 3 + neg_binom(0.15) rv_put_dec 55.57 15.74 (6) > 3 + neg_binom(0.20) linux_put_dec 57.18 16.50 (7) > 3 + neg_binom(0.20) rv_put_dec 51.15 13.58 (12) > 3 + neg_binom(0.50) linux_put_dec 45.06 6.39 (4) > 3 + neg_binom(0.50) rv_put_dec 41.16 6.51 (6) > > > New code uses lookup table which implies cache misses. Current code > > is purely code. > > Code can miss the cache also, and then it needs to be decoded again. The > new _code_ is slightly smaller, although the total .text+.rodata does > increase by ~150 bytes. Yes, overall the new code will probably > touch one or two extra cache lines compared to the old - so there's a > tradeoff between one-shot and bulk decimal conversions. > > >> On a larger scale, perf shows that top, one of the big consumers of /proc > >> data, uses 0.5-1.0% fewer cpu cycles. > > > > perf(1) also shows variance next to average, what was it? > > Not in the output I got - that just showed lines such as > > 2.35% top [kernel.kallsyms] [k] num_to_str > > But I don't have much perf-fu, so maybe I should have invoked it differently. I thought you did macrobenchmark with "perf stat -r", then it shows variance. > > First number printing improvement patch was measuring ~30% speedups: > > commit 4277eedd7908a0ca8b66fad46ee76b0ad96e6ef2 > > vsprintf.c: optimizing, part 2: base 10 conversion speedup, v2 > > > > Now it is 1%. > > That's comparing apples and oranges. The ~30% were obtained from a top > which was modified to do nothing but read /proc/pid/stat, the 1% is out > of all the cycles top actually spends. > > I think any further improvements to number printing code should be rejected > > on philosophical grounds: > > Hm, "perfect is the enemy of good" and all that. > > > Kernel should ship numbers to ps(1) and top(1) in BINARY, > > so it would take exactly 1 MOV instruction which takes exactly 1 cycle > > to execute. > > Currently it is 1) kernel converts binary to text, 2) usespace > > converts text to binary, > > 3) userspace converts binary to text and shows the user. 4) people optimizing #1 > > I agree this is somewhat silly, but that's what we have, and it is > unlikely to change anytime soon. Adding a parallel binary interface > would be a maintenance nightmare. Hm, such interface already exists, it's called CONFIG_TASKSTATS. procps and top of course don't use it. Alexey