Re: RFC on io-stalls patch

[Chris Mason <mason@suse.com>]

On Mon, 2003-07-14 at 16:24, Andrea Arcangeli wrote:

> 
> this isn't what we had in pre4, this is more equivalent to an hack in
> pre4 where the requests aren't distributed anymore 50/50 but 10/90. Of
> course an I/O queue filled mostly by parallel sync reads, will make the
> writer go slower since it will very rarely find the queue not oversized
> in presence of a flood of readers. So the writer won't be able to make
> progress.
> 
> This is a "stop the writers and give unlimited requests to the reader"
> hack, not a "reserve some request for the reader", of course then the
> reader is faster. of course then, contest shows huge improvements for
> the read loads.

Which is why it's a good place to start.  If we didn't see huge
improvements here, there's no use in experimenting with this tactic
further.

> 
> But contest only benchmarks the reader, it has no clue how fast the
> writer is AFIK. I would love to hear the bandwidth the writer is writing
> into the xtar_load. Maybe it's shown in some of the Loads/LCPU or
> something, but I don't know the semantics of those fields and I only
> look at time and ratio which I understand. so if any contest expert can
> enaborate if the xtar_load bandwidth is taken into consideration
> somewhere I would love to hear.

I had a much longer reply at first as well, but this is only day 1 of
Jens' benchmark, and he had plans to test other workloads.  I'd like to
give him the chance to do that work before we think about merging the
patch.  It's a good starting point for the question "can we do better
for reads?" (clearly the answer is yes).

-chris