Re: RFC on io-stalls patch

[Andrea Arcangeli <andrea@suse.de>]

On Tue, Jul 15, 2003 at 07:45:51AM +0200, Jens Axboe wrote:
> On Tue, Jul 15 2003, Andrea Arcangeli wrote:
> > On Mon, Jul 14, 2003 at 05:52:38PM -0700, Andrew Morton wrote:
> > > Chris Mason <mason@suse.com> wrote:
> > > >
> > > > If we go back to Jens' numbers:
> > > > 
> > > > ctar_load:
> > > > Kernel            [runs]        Time    CPU%    Loads   LCPU%   Ratio
> > > > 2.4.22-pre5            3        235     114.0   25.0    22.1    1.75
> > > > 2.4.22-pre5-axboe      3        194     138.1   19.7    20.6    1.46
> > > >                                                ^^^^^^
> > > > The loads column is the number of times ctar_load managed to run during
> > > > the kernel compile, and the patched kernel loses each time.  This must
> > > > partially be caused by the lower run time overall, but it is still
> > > > important data.  It would be better if contest gave us some kind of
> > > > throughput numbers (avg load time or something).
> > > 
> > > Look at the total CPU utilisation.  It went from 136% to 159% while both
> > > loads made reasonable progress.  Goodness.
> > 
> > if you look at the cpu utilization, stopping more the writer will
> > generate a cpu utilization even higher, would you mind if Loads shows 15
> 
> Correct
> 
> > instead of 19.7 so the CPU% can go from 138 to 148 and LCPU only goes
> > down from 20.6 to 18.8?  Problem is, how much should the writer be
> > stopped. The LCPU will be almost constant, it's I/O bound anyways. So
> > the more you stop the writer the higher the global cpu utilization will
> > be. This doesn't automatically mean goodness.
> 
> The above case is pretty much only goodness though, ratio of loads/time
> unit is about the same and we complete the workload much quicker
> (because of the higher cpu util).

the I/O write throughput takes a -24% hit.  The kernel compile runs 21%
faster.

Note that -24% is the opposite of +31%.

Problem is that we can't easily compare I/O bandwidth with cpu
utilization. But slowing down the writer of 31% of its previous
bandwidth, doesn't look an obvious improvement despite the kernel
compiles 21% faster. Depends if you were waiting on the write, or on the
kernel compile.

> I don't even think that is necessary, I feel fine with just the single
> queue free list. I just want to make sure that some reads can get in,
> while the queue maintains flooded by writes 99.9% of the time (trivial

that's ok.

> scenario, unlike the 'read starving all writers, might as well SIGSTOP
> tar' work load you talk about).

it's still not completely obvious if your improvement didn't came partly
from the SIGSTOP tar. Probably not because we found later that contest
uses nr_cpus * 4 and not some bigger number that I expected.

Andrea