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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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