Re: refill_buffers has high CPU utilization

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

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.

-- 
Jens Axboe