Wednesday, September 14, 2011

SD Card Speeds

After yesterday's benchmarks revealed a possible problem with the SD card performance, Mark Salter asked me to run a few more tests to see if the DMA API patch set is hurting the SD card (but helping USB storage).

I recompiled the 2.6.40.3-2.02.fc13.armv7l.omap kernel but I disabled the DMA API patch set.  I also tested with smp (the default) and nosmp.  And just for good measure, I re-tested the 2.6.35-g6d019da-dirty kernel because the last time I tested 2.6.35 was in May and I've updated a few packages since then.

It does not appear that the DMA API patch set is to blame for the slower SD card performance.  The random seek times were longer than 76000ms for all variants of 2.6.40.3 compared to just 664ms on 2.6.35.  The sequential reads were fastest with 2.6.35, but today's test was quite a bit faster than May's test; I'm not sure what to make of that.  Likewise, sequential writes were also fastest with 2.6.35.

Thus, some other change between 2.6.35 and 2.6.40 must be responsible for the drop in SD card performance, but what?

Version 1.96Sequential OutputSequential InputRandom
Seeks
SizePer CharBlockRewritePer CharBlock
K/sec% CPUK/sec% CPUK/sec% CPUK/sec% CPUK/sec% CPU/sec% CPU
SD Card 2.6.35 (May 2011)2G4193312553371433699175981527.41
Latency1168ms33487ms32391ms44466us2628ms663ms
SD Card 2.6.35 (Sept 2011)2G3991311853465444899191041627.91
Latency3900ms42424ms54958ms19929us667ms664ms
SD Card 2.6.40.3 + DMA patch + SMP2G2265186242544333999196261615.61
Latency2793ms29928ms39111ms50446us665ms76023ms
SD Card 2.6.40.3 + DMA patch + nosmp2G1958166932513334399208291317.21
Latency2858ms38268ms42161ms51148us667ms76651ms
SD Card 2.6.40.3 + no patch + SMP2G2676166932271341199195921615.71
Latency2855ms34616ms58124ms22279us666ms82254ms
SD Card 2.6.40.3 + no patch + nosmp2G1854160332395338299208941316.51
Latency5020ms33958ms29546ms51301us666ms77587ms

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