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SubjectRe: [PATCHSET] Linux 2.4.18-rc3-jam1
From
> > Random Writes
> > Num Avg Maximum Lat% Lat% CPU
> > Kernel Thr Rate (CPU%) Latency Latency >2s >10s Eff
> > ----------------- --- ------------------------------------------------------
> > 2.4.18-rc2 128 0.67 1.87% 1.334 777.23 0.00000 0.00000 36
> > 2.4.18-rc2-jam1 128 0.80 5.72% 0.190 3.68 0.00000 0.00000 14
> > 2.4.18rc2aa2 128 0.61 1.39% 61.796 72674.58 0.32761 0.32761 44
>
> Holy cow! Are you sure these numbers are right?

Max latency for random writes at 128 threads varies widely. The numbers
above happen to hit on the widest variation.

Random Writes
Num Avg Maximum Lat% Lat% CPU
Kernel Thr Rate (CPU%) Latency Latency >2s >10s Eff
----------------- --- ------------------------------------------------------
2.4.17-rmap12e 128 0.71 5.19% 1.689 981.25 0.00000 0.00000 14
2.4.17-rmap12f 128 0.71 4.97% 0.975 917.19 0.00000 0.00000 14
2.4.18-pre9 128 0.70 1.82% 1.664 785.13 0.00000 0.00000 38
2.4.18-pre9-ac1 128 0.67 1.93% 0.339 398.63 0.00000 0.00000 35
2.4.18-pre9-ac3 128 0.71 5.09% 0.438 951.04 0.00000 0.00000 14
2.4.18-pre9-ac4 128 0.72 5.12% 0.191 11.63 0.00000 0.00000 14
2.4.18-pre9-am1 128 0.76 2.72% 1.527 845.77 0.00000 0.00000 28
2.4.18-pre9-am2 128 0.75 2.83% 1.371 886.19 0.00000 0.00000 27
2.4.18-pre9-mjc2 128 0.71 3.72% 0.220 12.10 0.00000 0.00000 19
2.4.18-rc1 128 0.68 1.81% 1.584 800.21 0.00000 0.00000 37
2.4.18-rc1-slb1 128 0.81 5.77% 0.192 5.25 0.00000 0.00000 14
2.4.18-rc2 128 0.67 1.87% 1.334 777.23 0.00000 0.00000 36
2.4.18-rc2-ac2 128 0.71 5.19% 0.340 555.11 0.00000 0.00000 14
2.4.18-rc2-jam1 128 0.80 5.72% 0.190 3.68 0.00000 0.00000 14
2.4.18-rc4-jam1 128 0.80 5.72% 5.025 6734.19 0.07560 0.00000 14
2.4.18rc2aa2 128 0.61 1.39% 61.796 72674.58 0.32761 0.32761 44
2.5.3-dj5-tio 128 0.75 2.17% 0.209 1.73 0.00000 0.00000 34
2.5.4-dj2 128 0.76 2.23% 0.206 1.83 0.00000 0.00000 34
2.5.4-dj3 128 0.75 2.30% 0.207 1.87 0.00000 0.00000 33
2.5.5 128 0.74 2.09% 0.197 1.97 0.00000 0.00000 35
2.5.5-dj1 128 0.76 2.26% 0.201 1.45 0.00000 0.00000 33
2.5.5-pre1 128 0.75 2.28% 0.197 2.64 0.00000 0.00000 33

> The increased throughput will be thanks to the boosted request
> queue size.
>
> The (greatly) increased CPU load will also be due to browsing the eight-times
> larger request queue. Plus we browse it a bit more than we used to.
>
> The improvement in worst-case latency in both -aa and -jam will
> be due to the FIFO wait for requests.

Thanks for the insight there.

> But improvement by a factor of 20,000 sounds a little excessive :)
> And a maximum latency of three milliseconds would seem to indicate
> that the benchmark is *never* waiting on disk seek, and that
> perhaps the request queue is simply never filling up. But that
> doesn't make sense.

I notice the 2.5.x kernels also have extremely low max latency on this test.

> What does the "latency" actually mean? Is it the time spent
> in the kernel to issue a write(2)?

AFAICT, it's the time between when a request like write(2) is made,
and when it completes. It doesn't appear to be actual time in kernel.
I'd rather hear from a more knowledgeable coder though.

> Something funny is happening, I suspect. Guess I should go
> look at tiobench...

Cool. I admire your work.

Below is a snippet of tiobench on random writes. The rc4-jam1
included the entire patchset, whereas rc2-jam1 had patches with
the first two digits < 20.

Most notable is that jam1 had an unusual low max latency at
16 threads, which didn't appear at 8, 32, 64, and 128.

Random Writes
Num Avg Maximum Lat% Lat% CPU
Kernel Thr Rate (CPU%) Latency Latency >2s >10s Eff
-------------------- ------------------------------------------------------
2.4.18-rc2-jam1 8 0.71 4.71% 0.190 3.53 0.00000 0.00000 15
2.4.18-rc4-jam1 8 0.71 4.61% 0.754 2242.97 0.02500 0.00000 15

2.4.18-rc2-jam1 16 0.72 4.76% 0.192 4.02 0.00000 0.00000 15
2.4.18-rc4-jam1 16 0.72 4.72% 0.196 5.09 0.00000 0.00000 15

2.4.18-rc2-jam1 32 0.77 5.13% 0.192 5.28 0.00000 0.00000 15
2.4.18-rc4-jam1 32 0.77 5.25% 8.268 13672.38 0.15000 0.00000 15

2.4.18-rc2-jam1 64 0.79 5.27% 0.190 3.63 0.00000 0.00000 15
2.4.18-rc4-jam1 64 0.78 5.30% 26.660 27314.64 0.50403 0.02520 15

2.4.18-rc2-jam1 128 0.80 5.72% 0.190 3.68 0.00000 0.00000 14
2.4.18-rc4-jam1 128 0.80 5.72% 5.025 6734.19 0.07560 0.00000 14

--
Randy Hron

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