Re: RFC on io-stalls patch

[Jens Axboe]

On Tue, Jul 15 2003, Andrea Arcangeli wrote:
> On Mon, Jul 14, 2003 at 05:52:38PM -0700, Andrew Morton wrote:
> > Chris Mason <[e-mail]> 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).

> But my argument is that a patch that can generate indefinite starvation
> for every writer (given enough parallel sync reades), and that can as
> well lock into ram an excessive amount of ram (potentially all ram in
> the box) isn't goodness from my point of view.

Yes that one has been stated a few times.

> Having a separate read queue, limited in bytes, sounds ok instead,
> especially if it can generate results like the above. Heuristics
> optimizing common cases are fine, as far as they're safe for the corner
> cases too.

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

-- 
Jens Axboe