On Thu, Mar 25 2010, Jens Axboe wrote: > On Thu, Mar 25 2010, Veal, Bryan E wrote: > > Hi all, > > > > I'm experiencing really high CPU utilization with the refill_buffers option, presumably due to using rand() to generate all the data: > > > > Output with zero_buffers: > > zero_buffers: (g=0): rw=randwrite, bs=64K-64K/64K-64K, ioengine=psync, iodepth=1 > > ... > > zero_buffers: (g=0): rw=randwrite, bs=64K-64K/64K-64K, ioengine=psync, iodepth=1 > > zero_buffers: (groupid=0, jobs=32): err= 0: pid=21556 > > write: io=4600MB, bw=156966KB/s, iops=2452, runt= 30009msec > > clat (usec): min=378, max=139675, avg=13045.49, stdev=1468.67 > > bw (KB/s) : min= 2609, max= 6677, per=3.11%, avg=4886.17, stdev=120.46 > > cpu : usr=0.30%, sys=1.87%, ctx=2452182, majf=0, minf=11463 > > > > Output with refill_buffers: > > refill_buffers: (g=0): rw=randwrite, bs=64K-64K/64K-64K, ioengine=psync, iodepth=1 > > ... > > refill_buffers: (g=0): rw=randwrite, bs=64K-64K/64K-64K, ioengine=psync, iodepth=1 > > refill_buffers: (groupid=0, jobs=32): err= 0: pid=21503 > > write: io=4246MB, bw=144867KB/s, iops=2263, runt= 30010msec > > clat (usec): min=293, max=140908, avg=13969.29, stdev=1837.85 > > bw (KB/s) : min= 1187, max= 6843, per=3.13%, avg=4535.65, stdev=204.58 > > cpu : usr=37.76%, sys=1.63%, ctx=2286876, majf=0, minf=29750 > > > > While it is useful to write random data, the overhead is prohibitively > > expensive in high-throughput tests. Would it be a better option to > > allocate a large memory buffer, initialize it with random data, and > > use random offsets within the buffer for data to write to the disk? > > I think we should improve it, yes. I like the concept of the data being > pseudo random and non-repetitive at least, since that is guaranteed not > to be compressible. But it doesn't have to be cryptographically strong > by any means, so it should be pretty easy to have a in-fio rand() that > is fast yet good enough for the purpose. > 30% utilization just for > generating random buffers at a fairly slow rate of ~140MB/sec is > definitely excessive and not appropriate. > > I'll see to fixing that. I took a quick stab at it, and stole a rand implementation from networking. Net result here on the laptop is that it's 3x faster, a null write test goes from ~500MB/sec to ~1500MB/sec. I'd still like it to be much faster than this, so perhaps some pre-generated data with a bit of shuffling could still improve on this. Can you rerun your above test and see what the result is like now, if you pull or download the latest snapshot? -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
