Mark,
On Sat, 12 Aug 2000, Mark Hahn wrote:
> > > readahead is effecting things (shouldnt, but...)
> >
> > mke2fs only allows block sizes of 1K, 2K or 4K, so I can't make the
> > blocksize any larger...
>
> I meant "size of block being read", rather than filesystem block size.
I've now tested it with 2.2.16 and 8k block reading size - sorry about the
misunderstanding...
Dir Size BlkSz Thr# Read (CPU%) Write (CPU%) Seeks (CPU%)
----- ------ ------- ---- ------------- -------------- --------------
/mnt/ 256 8192 1 26.8839 10.2% 25.6472 22.4% 141.201 1.07%
/mnt/ 256 8192 2 20.4838 8.40% 25.6120 22.8% 136.777 0.80%
/mnt/ 256 8192 4 13.1292 6.09% 25.5942 23.0% 137.310 0.72%
/mnt/ 256 8192 8 10.5354 5.56% 25.4997 23.3% 139.946 0.74%
/mnt/ 256 8192 16 8.48961 5.23% 25.4394 23.4% 142.674 0.81%
/mnt/ 256 8192 32 6.97233 5.51% 25.2732 23.3% 144.932 0.91%
This doesn't appear to make things any better...
> > > so when this is running, does "vmstat 1" indicate that the system's
> > > swapping? I'm guessing so, that it's trying to scavenge pages from
> > > live processes, ineffectually (since they're live), and thrashing.
> >
> > After going to all the trouble of testing this out, I remembered that I
> > had not allocated any swap so that it couldn't influence the results at
> > all! d'oh!
>
> Linux is designed to have swap. I doubt anyone cares about how it
> behaves if you cripple it.
Since this is designed to test raw disk performance, I wanted to reduce
any other factors that might influence it. This includes reducing disk
caching/buffering (by lowering memory) other disk activity (removing
swap). It means I know it's not thrashing pages.
The fact remains that disk performance is much worse under 2.2.16 and
heavy loads than under 2.2.15 - what I was trying to find out was what
was causing it. Turning off swap under 2.2.15 doesn't seem to affect
the performance at all, so /something/ has still gone wrong with the
disk subsystem.
The other reason I've turned off swap is because we don't generally use it
with our RAID boxes - there are still a number or really serious issues
running RAID and swap - and if you run just the ext2 partitions on RAID,
then the machine will still die with a failed disk if it swaps!
> > I think I've located the problem... (with kernel 2.4, anyway) - it is
> > refusing to use DMA.
>
> CONFIG_ option.
Thanks, I have corrected my mistake now. Disk read performance (compared
to 2.2.16) is down for one thread, but decays much less quickly as threads
scale up.
> > hdc: [PTBL] [4982/255/63] hdc1
>
> geometry, initially read from bios and/or disk, is overridden by
> values inferred from the partition table.
Thanks.
> > Anybody have any suggestions about how to get DMA working? Is it a problem
> > with the IDE controller?
>
> you didn't choose exactly the right ide CONFIG_ options.
Yup, that's the cause behind the 2.4 probs.
Regards,
Corin
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