Re: [PATCH-RFC] cfq: Disable low_latency by default for 2.6.32

Corrado Zoccolo
2009-11-27T12:03:33Z

On Fri, Nov 27, 2009 at 12:44 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> On Thu, Nov 26, 2009 at 04:18:18PM +0100, Corrado Zoccolo wrote:
>> > <SNIP>
>> >
>> > In case you mean a partial disabling of cfq_latency, I'm try the
>> > following patch. The intention is to disable the low_latency logic if
>> > kswapd is at work and presumably needs clean pages. Alternative
>> > suggestions welcome.
>
> As it turned out, that patch sucked so I aborted the test and I need to
> think about it a lot more.
What about using the dirty ratio, instead of checking if kswapd is running?

>> Yes, I meant exactly to disable that part, and doing it when kswapd is
>> active is probably a good choice.
>> I have a different idea for 2.6.33, though.
>> If you have a reliable reproducer of the issue, can you test it on
>> git://git.kernel.dk/linux-2.6-block.git branch for-2.6.33?
>> It may already be unaffected, since we had various performance
>> improvements there, but I think a better way to boost writeback is
>> possible.
>>
>
> I haven't tested the high-order allocation scenario yet but the results
> as thing stands are below. There are four kernels being compared
>
> 1. with-low-latency               is 2.6.32-rc8 vanilla
> 2. with-low-latency-block-2.6.33  is with the for-2.6.33 from linux-block applied
> 3. with-low-latency-async-rampup  is with "[RFC,PATCH] cfq-iosched: improve async queue ramp up formula"
> 4. without-low-latency            is with low_latency disabled
>
> SYSBENCH
>                 sysbench-with       low-latency       low-latency  sysbench-without
>                   low-latency      block-2.6.33      async-rampup       low-latency
>           1  1266.02 ( 0.00%)   824.08 (-53.63%)  1265.15 (-0.07%)  1278.55 ( 0.98%)
>           2  1182.58 ( 0.00%)  1226.42 ( 3.57%)  1223.03 ( 3.31%)  1379.25 (14.26%)
>           3  1218.64 ( 0.00%)  1271.38 ( 4.15%)  1246.42 ( 2.23%)  1580.08 (22.87%)
>           4  1212.11 ( 0.00%)  1257.84 ( 3.64%)  1325.17 ( 8.53%)  1534.17 (20.99%)
>           5  1046.77 ( 0.00%)   981.71 (-6.63%)  1008.44 (-3.80%)  1552.48 (32.57%)
>           6  1187.14 ( 0.00%)  1132.89 (-4.79%)  1147.18 (-3.48%)  1661.19 (28.54%)
>           7  1179.37 ( 0.00%)  1183.61 ( 0.36%)  1202.49 ( 1.92%)   790.26 (-49.24%)
>           8  1164.62 ( 0.00%)  1143.54 (-1.84%)  1184.56 ( 1.68%)   854.10 (-36.36%)
>           9  1095.22 ( 0.00%)  1178.72 ( 7.08%)  1002.42 (-9.26%)  1655.04 (33.83%)
>          10  1147.52 ( 0.00%)  1153.46 ( 0.52%)  1151.73 ( 0.37%)  1653.89 (30.62%)
>          11   823.38 ( 0.00%)   820.64 (-0.33%)   754.15 (-9.18%)  1627.45 (49.41%)
>          12   813.73 ( 0.00%)   791.44 (-2.82%)   848.32 ( 4.08%)  1494.63 (45.56%)
>          13   898.22 ( 0.00%)   789.63 (-13.75%)   931.47 ( 3.57%)  1521.64 (40.97%)
>          14   873.50 ( 0.00%)   938.90 ( 6.97%)   875.75 ( 0.26%)  1311.09 (33.38%)
>          15   808.32 ( 0.00%)   979.88 (17.51%)   877.87 ( 7.92%)  1009.70 (19.94%)
>          16   758.17 ( 0.00%)  1096.81 (30.87%)   881.23 (13.96%)   725.17 (-4.55%)
>
> sysbench is helped by both both block-2.6.33 and async-rampup to some
> extent. For many of the results, plain old disabling low_latency still
> helps the most.
>
> desktop-net-gitk
>                     gitk-with       low-latency       low-latency      gitk-without
>                   low-latency      block-2.6.33      async-rampup       low-latency
> min            954.46 ( 0.00%)   570.06 (40.27%)   796.22 (16.58%)   640.65 (32.88%)
> mean           964.79 ( 0.00%)   573.96 (40.51%)   798.01 (17.29%)   655.57 (32.05%)
> stddev          10.01 ( 0.00%)     2.65 (73.55%)     1.91 (80.95%)    13.33 (-33.18%)
> max            981.23 ( 0.00%)   577.21 (41.17%)   800.91 (18.38%)   675.65 (31.14%)
>
> The changes for block in 2.6.33 make a massive difference here, notably
> beating the disabling of low_latency.
Yes. These are read of lots of small files, so the improvements for
seeky workload we introduced in 2.6.33 helps a lot here.
>
> IOZone
>                           iozone-with           low-latency           low-latency        iozone-without
>                           low-latency          block-2.6.33          async-rampup           low-latency
> write-64               151212 ( 0.00%)       163359 ( 7.44%)       163359 ( 7.44%)       159856 ( 5.41%)
> write-128              189357 ( 0.00%)       184922 (-2.40%)       202805 ( 6.63%)       206233 ( 8.18%)
> write-256              219883 ( 0.00%)       211232 (-4.10%)       189867 (-15.81%)       223174 ( 1.47%)
> write-512              224932 ( 0.00%)       222601 (-1.05%)       204459 (-10.01%)       220227 (-2.14%)
> write-1024             227738 ( 0.00%)       226728 (-0.45%)       216009 (-5.43%)       226155 (-0.70%)
> write-2048             227564 ( 0.00%)       224167 (-1.52%)       229387 ( 0.79%)       224848 (-1.21%)
> write-4096             208556 ( 0.00%)       227707 ( 8.41%)       216908 ( 3.85%)       223430 ( 6.66%)
> write-8192             219484 ( 0.00%)       222365 ( 1.30%)       217737 (-0.80%)       219389 (-0.04%)
> write-16384            206670 ( 0.00%)       209355 ( 1.28%)       204146 (-1.24%)       206295 (-0.18%)
> write-32768            203023 ( 0.00%)       205097 ( 1.01%)       199766 (-1.63%)       201852 (-0.58%)
> write-65536            162134 ( 0.00%)       196670 (17.56%)       189975 (14.66%)       189173 (14.29%)
> write-131072            68534 ( 0.00%)        69145 ( 0.88%)        64519 (-6.22%)        67417 (-1.66%)
> write-262144            32936 ( 0.00%)        28587 (-15.21%)        31470 (-4.66%)        27750 (-18.69%)
> write-524288            24044 ( 0.00%)        23560 (-2.05%)        23116 (-4.01%)        23759 (-1.20%)
> rewrite-64             755681 ( 0.00%)       800767 ( 5.63%)       469931 (-60.81%)       755681 ( 0.00%)
> rewrite-128            581518 ( 0.00%)       639723 ( 9.10%)       591774 ( 1.73%)       799840 (27.30%)
> rewrite-256            639427 ( 0.00%)       710511 (10.00%)       666414 ( 4.05%)       659861 ( 3.10%)
> rewrite-512            669577 ( 0.00%)       743788 ( 9.98%)       692017 ( 3.24%)       684954 ( 2.24%)
> rewrite-1024           680960 ( 0.00%)       755195 ( 9.83%)       701422 ( 2.92%)       686182 ( 0.76%)
> rewrite-2048           685263 ( 0.00%)       743123 ( 7.79%)       703445 ( 2.58%)       692780 ( 1.09%)
> rewrite-4096           631352 ( 0.00%)       686776 ( 8.07%)       640007 ( 1.35%)       643266 ( 1.85%)
> rewrite-8192           442146 ( 0.00%)       474089 ( 6.74%)       457768 ( 3.41%)       442624 ( 0.11%)
> rewrite-16384          428641 ( 0.00%)       454857 ( 5.76%)       442896 ( 3.22%)       432613 ( 0.92%)
> rewrite-32768          425361 ( 0.00%)       444206 ( 4.24%)       434472 ( 2.10%)       430568 ( 1.21%)
> rewrite-65536          405183 ( 0.00%)       433898 ( 6.62%)       419843 ( 3.49%)       389242 (-4.10%)
> rewrite-131072          66110 ( 0.00%)        58370 (-13.26%)        54342 (-21.66%)        58472 (-13.06%)
> rewrite-262144          29254 ( 0.00%)        24665 (-18.61%)        25710 (-13.78%)        29306 ( 0.18%)
> rewrite-524288          23812 ( 0.00%)        20742 (-14.80%)        22490 (-5.88%)        24543 ( 2.98%)
> read-64                934589 ( 0.00%)      1160938 (19.50%)      1004538 ( 6.96%)       840903 (-11.14%)
> read-128              1601534 ( 0.00%)      1869179 (14.32%)      1681806 ( 4.77%)      1280633 (-25.06%)
> read-256              1255511 ( 0.00%)      1526887 (17.77%)      1304314 ( 3.74%)      1310683 ( 4.21%)
> read-512              1291158 ( 0.00%)      1377278 ( 6.25%)      1336145 ( 3.37%)      1319723 ( 2.16%)
> read-1024             1319408 ( 0.00%)      1306564 (-0.98%)      1368162 ( 3.56%)      1347557 ( 2.09%)
> read-2048             1316016 ( 0.00%)      1394645 ( 5.64%)      1339827 ( 1.78%)      1347393 ( 2.33%)
> read-4096             1253710 ( 0.00%)      1307525 ( 4.12%)      1247519 (-0.50%)      1251882 (-0.15%)
> read-8192              995149 ( 0.00%)      1033337 ( 3.70%)      1016944 ( 2.14%)      1011794 ( 1.65%)
> read-16384             883156 ( 0.00%)       905213 ( 2.44%)       905213 ( 2.44%)       897458 ( 1.59%)
> read-32768             844368 ( 0.00%)       855213 ( 1.27%)       849609 ( 0.62%)       856364 ( 1.40%)
> read-65536             816099 ( 0.00%)       839262 ( 2.76%)       835019 ( 2.27%)       826473 ( 1.26%)
> read-131072            818055 ( 0.00%)       837369 ( 2.31%)       828230 ( 1.23%)       824351 ( 0.76%)
> read-262144            827225 ( 0.00%)       839635 ( 1.48%)       840538 ( 1.58%)       835693 ( 1.01%)
> read-524288             24653 ( 0.00%)        21387 (-15.27%)        20602 (-19.66%)        22519 (-9.48%)
> reread-64             2329708 ( 0.00%)      2251544 (-3.47%)      1985134 (-17.36%)      1985134 (-17.36%)
> reread-128            1446222 ( 0.00%)      1979446 (26.94%)      2009076 (28.02%)      2137031 (32.33%)
> reread-256            1828508 ( 0.00%)      2006158 ( 8.86%)      1892980 ( 3.41%)      1879725 ( 2.72%)
> reread-512            1521718 ( 0.00%)      1642783 ( 7.37%)      1508887 (-0.85%)      1579934 ( 3.68%)
> reread-1024           1347557 ( 0.00%)      1422540 ( 5.27%)      1384034 ( 2.64%)      1375171 ( 2.01%)
> reread-2048           1340664 ( 0.00%)      1413929 ( 5.18%)      1372364 ( 2.31%)      1350783 ( 0.75%)
> reread-4096           1259592 ( 0.00%)      1324868 ( 4.93%)      1273788 ( 1.11%)      1284839 ( 1.96%)
> reread-8192           1007285 ( 0.00%)      1033710 ( 2.56%)      1027159 ( 1.93%)      1011317 ( 0.40%)
> reread-16384           891404 ( 0.00%)       910828 ( 2.13%)       916562 ( 2.74%)       905022 ( 1.50%)
> reread-32768           850492 ( 0.00%)       859341 ( 1.03%)       856385 ( 0.69%)       862772 ( 1.42%)
> reread-65536           836565 ( 0.00%)       852664 ( 1.89%)       852315 ( 1.85%)       847020 ( 1.23%)
> reread-131072          844516 ( 0.00%)       862590 ( 2.10%)       854067 ( 1.12%)       853155 ( 1.01%)
> reread-262144          851524 ( 0.00%)       860559 ( 1.05%)       864921 ( 1.55%)       860653 ( 1.06%)
> reread-524288           24927 ( 0.00%)        21300 (-17.03%)        19748 (-26.23%)        22487 (-10.85%)
> randread-64           1605256 ( 0.00%)      1605256 ( 0.00%)      1605256 ( 0.00%)      1775099 ( 9.57%)
> randread-128          1179358 ( 0.00%)      1582649 (25.48%)      1511363 (21.97%)      1528576 (22.85%)
> randread-256          1421755 ( 0.00%)      1599680 (11.12%)      1460430 ( 2.65%)      1310683 (-8.47%)
> randread-512          1306873 ( 0.00%)      1278855 (-2.19%)      1243315 (-5.11%)      1281909 (-1.95%)
> randread-1024         1201314 ( 0.00%)      1254656 ( 4.25%)      1190657 (-0.90%)      1231629 ( 2.46%)
> randread-2048         1179413 ( 0.00%)      1227971 ( 3.95%)      1185272 ( 0.49%)      1190529 ( 0.93%)
> randread-4096         1107005 ( 0.00%)      1160862 ( 4.64%)      1110727 ( 0.34%)      1116792 ( 0.88%)
> randread-8192          894337 ( 0.00%)       924264 ( 3.24%)       912676 ( 2.01%)       899487 ( 0.57%)
> randread-16384         783760 ( 0.00%)       800299 ( 2.07%)       793351 ( 1.21%)       791341 ( 0.96%)
> randread-32768         740498 ( 0.00%)       743720 ( 0.43%)       741233 ( 0.10%)       743511 ( 0.41%)
> randread-65536         721640 ( 0.00%)       727692 ( 0.83%)       726984 ( 0.74%)       728139 ( 0.89%)
> randread-131072        715284 ( 0.00%)       722094 ( 0.94%)       717746 ( 0.34%)       720825 ( 0.77%)
> randread-262144        709855 ( 0.00%)       706770 (-0.44%)       709133 (-0.10%)       714943 ( 0.71%)
> randread-524288           394 ( 0.00%)          421 ( 6.41%)          418 ( 5.74%)          431 ( 8.58%)
> randwrite-64           730988 ( 0.00%)       764288 ( 4.36%)       723111 (-1.09%)       730988 ( 0.00%)
> randwrite-128          746459 ( 0.00%)       799840 ( 6.67%)       746459 ( 0.00%)       742331 (-0.56%)
> randwrite-256          695778 ( 0.00%)       752329 ( 7.52%)       720041 ( 3.37%)       727850 ( 4.41%)
> randwrite-512          666253 ( 0.00%)       722760 ( 7.82%)       667081 ( 0.12%)       691126 ( 3.60%)
> randwrite-1024         651223 ( 0.00%)       697776 ( 6.67%)       663292 ( 1.82%)       659625 ( 1.27%)
> randwrite-2048         655558 ( 0.00%)       691887 ( 5.25%)       665720 ( 1.53%)       664073 ( 1.28%)
> randwrite-4096         635556 ( 0.00%)       662721 ( 4.10%)       643170 ( 1.18%)       642400 ( 1.07%)
> randwrite-8192         467357 ( 0.00%)       491364 ( 4.89%)       476720 ( 1.96%)       469734 ( 0.51%)
> randwrite-16384        413188 ( 0.00%)       427521 ( 3.35%)       417353 ( 1.00%)       417282 ( 0.98%)
> randwrite-32768        404161 ( 0.00%)       411721 ( 1.84%)       404942 ( 0.19%)       407580 ( 0.84%)
> randwrite-65536        379372 ( 0.00%)       397312 ( 4.52%)       386853 ( 1.93%)       381273 ( 0.50%)
> randwrite-131072        21780 ( 0.00%)        16924 (-28.69%)        21177 (-2.85%)        19758 (-10.23%)
> randwrite-262144         6249 ( 0.00%)         5548 (-12.64%)         6370 ( 1.90%)         6316 ( 1.06%)
> randwrite-524288         2915 ( 0.00%)         2582 (-12.90%)         2871 (-1.53%)         2859 (-1.96%)
> bkwdread-64           1141196 ( 0.00%)      1141196 ( 0.00%)      1004538 (-13.60%)      1141196 ( 0.00%)
> bkwdread-128          1066865 ( 0.00%)      1386465 (23.05%)      1400936 (23.85%)      1101900 ( 3.18%)
> bkwdread-256           877797 ( 0.00%)      1105556 (20.60%)      1105556 (20.60%)      1105556 (20.60%)
> bkwdread-512          1133103 ( 0.00%)      1162547 ( 2.53%)      1175271 ( 3.59%)      1162547 ( 2.53%)
> bkwdread-1024         1163562 ( 0.00%)      1206714 ( 3.58%)      1213534 ( 4.12%)      1195962 ( 2.71%)
> bkwdread-2048         1163439 ( 0.00%)      1218910 ( 4.55%)      1204552 ( 3.41%)      1204552 ( 3.41%)
> bkwdread-4096         1116792 ( 0.00%)      1175477 ( 4.99%)      1159922 ( 3.72%)      1150600 ( 2.94%)
> bkwdread-8192          912288 ( 0.00%)       935233 ( 2.45%)       944695 ( 3.43%)       934724 ( 2.40%)
> bkwdread-16384         817707 ( 0.00%)       824140 ( 0.78%)       832527 ( 1.78%)       829152 ( 1.38%)
> bkwdread-32768         775898 ( 0.00%)       773714 (-0.28%)       785494 ( 1.22%)       787691 ( 1.50%)
> bkwdread-65536         759643 ( 0.00%)       769924 ( 1.34%)       778780 ( 2.46%)       772174 ( 1.62%)
> bkwdread-131072        763215 ( 0.00%)       769634 ( 0.83%)       773707 ( 1.36%)       773816 ( 1.37%)
> bkwdread-262144        765491 ( 0.00%)       768992 ( 0.46%)       780876 ( 1.97%)       780021 ( 1.86%)
> bkwdread-524288          3688 ( 0.00%)         3595 (-2.59%)         3577 (-3.10%)         3724 ( 0.97%)
>
> The upcoming changes for 2.6.33 also help iozone in many cases, often by more
> than just disabling low_latency. It has the occasional massive gain or loss
> for the larger file sizes. I don't know why this is but as the big losses
> appear to be mostly in the write-tests, I would guess that it's differences
> in heavy-writer-throttling.
I wonder if 2.6.33 + my async rampup patch will improve still further,
maybe reaching the low_latency=0 performance also for writing tests.
>
> The only downside with block-2.6.33 is that there are a lot of patches in
> there and doesn't help with the 2.6.32 release as such. I could do a reverse
> bisect to see what helps the most in there but under ideal conditions, it'll
> take 3 days to complete and I wouldn't be able to start until Monday as I'm
> out of the country for the weekend. That's a bit late.
Bisect will likely not help, since we have several patch series with
heavy internal dependencies in that tree.
If one of the patch series is found to bring the improvement, you have
to backport the entire series, that is not advisable for a rc8 or for
stable.
>
> p.s. As a consequence of being out of the country, I also won't be able to
>     respond to mail over the weekend.
>
> --
> Mel Gorman
>
Thanks for the detailed report
Corrado