Re: Performance regression in IO scheduler still there

Jan Kara
2009-11-16T16:58:01Z

On Mon 16-11-09 11:47:44, Jan Kara wrote:
> On Thu 12-11-09 15:44:02, Jeff Moyer wrote:
> > Jan Kara <jack@suse.cz> writes:
> >
> > > On Wed 11-11-09 12:43:30, Jeff Moyer wrote:
> > >> Jan Kara <jack@suse.cz> writes:
> > >>
> > >> > Sadly, I don't see the improvement you can see :(. The numbers are the
> > >> > same regardless low_latency set to 0:
> > >> > 2.6.32-rc5 low_latency = 0:
> > >> > 37.39 36.43 36.51 -> 36.776667 0.434920
> > >> > But my testing environment is a plain SATA drive so that probably
> > >> > explains the difference...
> > >>
> > >> I just retested (10 runs for each kernel) on a SATA disk with no NCQ
> > >> support and I could not see a difference. I'll try to dig up a disk
> > >> that support NCQ. Is that what you're using for testing?
> > > I don't think I am. How do I find out?
> >
> > Good question. ;-) I grep for NCQ in dmesg output and make sure it's
> > greater than 0/32. There may be a better way, though.
> Message in the logs:
> ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 300)
> ata1.00: ATA-8: Hitachi HTS722016K9SA00, DCDOC54P, max UDMA/133
> ata1.00: 312581808 sectors, multi 16: LBA48 NCQ (depth 0/32)
> ata1.00: configured for UDMA/133
> So apparently no NCQ. /sys/block/sda/device/queue_depth shows 1 but I
> guess that's just it's way of saying "no NCQ".
>
> What I thought might make a difference why I'm seeing the drop and you
> are not is size of RAM or number of CPUs vs the tiobench file size or
> number of threads. I'm running on a machine with 2 GB of RAM, using 4 GB
> filesize. The machine has 2 cores and I'm using 16 tiobench threads. I'm
> now rerunning tests with various numbers of threads to see how big
> difference it makes.
OK, here are the numbers (3 runs of each test):
2.6.29:
Threads Avg Stddev
1 42.043333 0.860439
2 40.836667 0.322938
4 41.810000 0.114310
8 40.190000 0.419603
16 39.950000 0.403072
32 39.373333 0.766913

2.6.32-rc7:
Threads Avg Stddev
1 41.580000 0.403072
2 39.163333 0.374641
4 39.483333 0.400111
8 38.560000 0.106145
16 37.966667 0.098770
32 36.476667 0.032998

So apparently the difference between 2.6.29 and 2.6.32-rc7 increases as
the number of threads rises. With how many threads have you been running
when using SATA drive and what machine is it?
I'm now running a test with larger file size (8GB instead of 4) to see
what difference it makes.

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR