Re:

Paul Turner
2011-07-22T00:58:09Z

On Thu, Jul 21, 2011 at 5:32 PM, Jason Baron <jbaron@redhat.com> wrote:
> 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.
>

Ah I should have mentioned that was one of the holes I stared down:

Builds were -O2 (gcc-4.6.1) and
$ zcat /proc/config.gz | grep CONFIG_CC_OPTIMIZE_FOR_SIZE
# CONFIG_CC_OPTIMIZE_FOR_SIZE is not set

Same kernel image across all platforms.

> thanks,
>
> -Jason
>