Messages in this thread | | | Date | Fri, 6 Nov 2009 00:00:40 +0100 | Subject | Re: Performance regression in IO scheduler still there | From | Corrado Zoccolo <> |
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Hi Jeff, what hardware are you using for tests? I see aggregated random read bandwidth is larger than sequential read bandwidth, and write bandwidth greater than read. Is this a SAN with multiple independent spindles?
On Thu, Nov 5, 2009 at 9:10 PM, Jeff Moyer <jmoyer@redhat.com> wrote: > Jan Kara <jack@suse.cz> writes: > >> Hi, >> >> I took time and remeasured tiobench results on recent kernel. A short >> conclusion is that there is still a performance regression which I reported >> few months ago. The machine is Intel 2 CPU with 2 GB RAM and plain SATA >> drive. tiobench sequential write performance numbers with 16 threads: >> 2.6.29: AVG STDERR >> 37.80 38.54 39.48 -> 38.606667 0.687475 >> >> 2.6.32-rc5: >> 37.36 36.41 36.61 -> 36.793333 0.408928 >> >> So about 5% regression. The regression happened sometime between 2.6.29 and >> 2.6.30 and stays the same since then... With deadline scheduler, there's >> no regression. Shouldn't we do something about it? > > Sorry it took so long, but I've been flat out lately. I ran some > numbers against 2.6.29 and 2.6.32-rc5, both with low_latency set to 0 > and to 1. Here are the results (average of two runs): > > rlat | rrlat | wlat | rwlat > kernel | Thr | read | randr | write | randw | avg, max | avg, max | avg, max | avg,max > ------------------------------------------------------------------------------------------------------------------------ > 2.6.29 | 8 | 72.95 | 20.06 | 269.66 | 231.59 | 6.625, 1683.66 | 23.241, 1547.97 | 1.761, 698.10 | 0.720, 443.64 > | 16 | 72.33 | 20.03 | 278.85 | 228.81 | 13.643, 2499.77 | 46.575, 1717.10 | 3.304, 1149.29 | 1.011, 140.30 > ------------------------------------------------------------------------------------------------------------------------ > 2.6.32-rc5 | 8 | 86.58 | 19.80 | 198.82 | 205.06 | 5.694, 977.26 | 22.559, 870.16 | 2.359, 693.88 | 0.530, 24.32 > | 16 | 86.82 | 21.10 | 199.00 | 212.02 | 11.010, 1958.78 | 40.195, 1662.35 | 4.679, 1351.27 | 1.007, 25.36 > ------------------------------------------------------------------------------------------------------------------------ > 2.6.32-rc5 | 8 | 87.65 | 117.65 | 298.27 | 212.35 | 5.615, 984.89 | 4.060, 97.39 | 1.535, 311.14 | 0.534, 24.29 > low_lat=0 | 16 | 95.60 | 119.95*| 302.48 | 213.27 | 10.263, 1750.19 | 13.899, 1006.21 | 3.221, 734.22 | 1.062, 40.40 > ------------------------------------------------------------------------------------------------------------------------ > > Legend: > rlat - read latency > rrlat - random read latency > wlat - write lancy > rwlat - random write latency > * - the two runs reported vastly different numbers: 67.53 and 172.46 > > So, as you can see, if we turn off the low_latency tunable, we get > better numbers across the board with the exception of random writes. > It's also interesting to note that the latencies reported by tiobench > are more favorable with low_latency set to 0, which is > counter-intuitive. > > So, now it seems we don't have a regression in sequential read > bandwidth, but we do have a regression in random read bandwidth (though > the random write latencies look better). So, I'll look into that, as it > is almost 10%, which is significant. >
Sorry, I don't see a 10% regression in random read from your numbers. I see a larger one in sequential write for low_latency=1 (this was the regression Jan reported in the original message), but not for low_latency=0. And a 10% regression in random writes, that is not completely fixed even by disabling low_latency.
I guess your seemingly counter-intuitive results for low_latency are due to the uncommon hardware (low_latency was intended mainly for desktop-class disks). Luckily, the patches queued for 2.6.33 already address this low_latency misbehaviour.
Thanks, Corrado.
> Cheers, > Jeff > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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