Re: refill_buffers has high CPU utilization

[Jens Axboe <jens.axboe@oracle.com>]

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