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rth@redhat.com
Bcc:
Subject: Re: [RFT][patch 17/18] sched: use jump labels to reduce overhead
when bandwidth control is inactive
Reply-To:
In-Reply-To: <20110721184758.403388616@google.com>

On Thu, Jul 21, 2011 at 09:43:42AM -0700, Paul Turner wrote:
> So I'm seeing some strange costs associated with jump_labels; while on paper
> the branches and instructions retired improves (as expected) we're taking an
> unexpected hit in IPC.
>
> [From the initial mail we have workloads:
> mkdir -p /cgroup/cpu/test
> echo $$ > /dev/cgroup/cpu/test (only cpu,cpuacct mounted)
> (W1) taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "for ((i=0;i<5;i++)); do $(dirname $0)/pipe-test 20000; done"
> (W2)taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "$(dirname $0)/pipe-test 100000;true"
> (W3)taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "$(dirname $0)/pipe-test 100000;"
> ]
>
> To make some of the figures more clear:
>
> Legend:
> !BWC = tip + bwc, BWC compiled out
> BWC = tip + bwc
> BWC_JL = tip + bwc + jump label (this patch)
>
>
> Now, comparing under W1 we see:
> W1: BWC vs BWC_JL
> instructions cycles branches elapsed
> ---------------------------------------------------------------------------------------------------------------------
> clovertown [BWC] 845934117 974222228 152715407 0.419014188 [baseline]
> +unconstrained 857963815 (+1.42) 1007152750 (+3.38) 153140328 (+0.28) 0.433186926 (+3.38) [rel]
> +10000000000/1000: 876937753 (+2.55) 1033978705 (+5.65) 160038434 (+3.59) 0.443638365 (+5.66) [rel]
> +10000000000/1000000: 880276838 (+3.08) 1036176245 (+6.13) 160683878 (+4.15) 0.444577244 (+6.14) [rel]
>
> barcelona [BWC] 820573353 748178486 148161233 0.342122850 [baseline]
> +unconstrained 817011602 (-0.43) 759838181 (+1.56) 145951513 (-1.49) 0.347462571 (+1.56) [rel]
> +10000000000/1000: 830109086 (+0.26) 770451537 (+1.67) 151228902 (+1.08) 0.350824677 (+1.65) [rel]
> +10000000000/1000000: 830196206 (+0.30) 770704213 (+2.27) 151250413 (+1.12) 0.350962182 (+2.28) [rel]
>
> westmere [BWC] 802533191 694415157 146071233 0.194428018 [baseline]
> +unconstrained 799057936 (-0.43) 751384496 (+8.20) 143875513 (-1.50) 0.211182620 (+8.62) [rel]
> +10000000000/1000: 812033785 (+0.27) 761469084 (+8.51) 149134146 (+1.09) 0.212149229 (+8.28) [rel]
> +10000000000/1000000: 811912834 (+0.27) 757842988 (+7.45) 149113291 (+1.09) 0.211364804 (+7.30) [rel]
> e.g. Barcelona issues ~0.43% less instructions, for a total of 817011602, in
> the unconstrained case with BWC.
>
>
> Where "unconstrained, 10000000000/1000, 10000000000/10000" are the on
> measurements for BWC_JL, with (%d) being the relative difference to their
> BWC counterparts.
>
> W1: BWC vs BWC_JL is very similar.
> BWC vs BWC_JL
> clovertown [BWC] 985732031 1283113452 175621212 1.375905653
> +unconstrained 979242938 (-0.66) 1288971141 (+0.46) 172122546 (-1.99) 1.389795165 (+1.01) [rel]
> +10000000000/1000: 999886468 (+0.33) 1296597143 (+1.13) 180554004 (+1.62) 1.392576770 (+1.18) [rel]
> +10000000000/1000000: 999034223 (+0.11) 1293925500 (+0.57) 180413829 (+1.39) 1.391041338 (+0.94) [rel]
>
> barcelona [BWC] 982139920 1078757792 175417574 1.069537049
> +unconstrained 965443672 (-1.70) 1075377223 (-0.31) 170215844 (-2.97) 1.045595065 (-2.24) [rel]
> +10000000000/1000: 989104943 (+0.05) 1100836668 (+0.52) 178837754 (+1.22) 1.058730316 (-1.77) [rel]
> +10000000000/1000000: 987627489 (-0.32) 1095843758 (-0.17) 178567411 (+0.84) 1.056100899 (-2.28) [rel]
>
> westmere [BWC] 918633403 896047900 166496917 0.754629182
> +unconstrained 914740541 (-0.42) 903906801 (+0.88) 163652848 (-1.71) 0.758050332 (+0.45) [rel]
> +10000000000/1000: 927517377 (-0.41) 952579771 (+5.67) 170173060 (+0.75) 0.771193786 (+2.43) [rel]
> +10000000000/1000000: 914676985 (-0.89) 936106277 (+3.81) 167683288 (+0.22) 0.764973632 (+1.38) [rel]
>
> Now this is rather odd, almost across the board we're seeing the expected
> drops in instructions and branches, yet we appear to be paying a heavy IPC
> price. The fact that wall-time has scaled equivalently with cycles roughly
> rules out the cycles counter being off.
>
> We are seeing the expected behavior in the bandwidth enabled case;
> specifically the <jl=jmp><ret><cond><ret> blocks are taking an extra branch
> and instruction which shows up on all the numbers above.
>
> With respect to compiler mangling the text is essentially unchanged in size.
> One lurking suspicion is whether the inserted nops have perturbed some of the
> jmp/branch alignments?
>
> text data bss dec hex filename
> 7277206 2827256 2125824 12230286 ba9e8e vmlinux.jump_label
> 7276886 2826744 2125824 12229454 ba9b4e vmlinux.no_jump_label
>
> I have checked to make sure that the right instructions are being patched in
> at run-time. I've also pulled a fully patched jump_label out of the kernel
> into a userspace test (and benchmarked it directly under perf). The results
> here are also exactly as expected.
>
> e.g.
> Performance counter stats for './jump_test':
> 1,500,839,002 instructions, 300,147,081 branches 702,468,404 cycles
> Performance counter stats for './jump_test 1':
> 2,001,014,609 instructions, 400,177,192 branches 901,758,219 cycles
>
> Overall if we can fix the IPC the benefit in the globally unconstrained case
> looks really good.
>
> Any thoughts Jason?
>

Do you have CONFIG_CC_OPTIMIZE_FOR_SIZE set? I know that when
CONFIG_CC_OPTIMIZE_FOR_SIZE is not set, the compiler can make the code
more optimal.

thanks,

-Jason


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