Re: [2.4.17/18pre] VM and swap - it's really unusable

[Andrew Morton]

Marcelo Tosatti wrote:
> 
> > Andrew Morten`s read-latency.patch is a clear winner for me, too.
> 
> AFAIK Andrew's code simply adds schedule points around the kernel, right?
> 
> If so, nope, I do not plan to integrate it.

I haven't sent it to you yet :)  It improves the kernel.  That's
good, isn't it?  (There are already forty or fifty open-coded
rescheduling points in the kernel.  That patch just adds the
missing (and most important) ten).  

BTW, with regard to the "preempt and low-lat improve disk throughput"
argument.  I have occasionally seen small throughput improvements,
but I think these may be just request-merging flukes.  Certainly
they were very small.

The one area where it sometimes makes a huuuuuge throughput
improvement is software RAID.

Much of the VM and dirty buffer writeout code assumes that
submit_bh() starts I/O.  Guess what?  RAID's submit_bh()
sometimes *doesn't* start I/O.  Because the IO is started
by a different thread.

With the Riel VM I had a test case in which software RAID
completely and utterly collapsed because of this.  The machine
was spending huge amounts of time spinning in page_launder(), madly
submitting I/O, but never yielding, so the I/O wasn't being started.

-aa VM has an open-coded yield in shrink_cahce() which prevents
that particular collapse.  But I had a report yesterday that
the mini-ll patch triples throughput on a complex RAID stack in
2.4.17.  Same reason.

Arguably, this is a RAID problem - raidN_make_request() should
be yielding.  But it's better to do this in one nice, single,
reviewable place - submit_bh().  However that won't prevent
wait_for_buffers() from starving the raid thread.

RAID is not alone.  ksoftirqd, keventd and loop_thread() also
need reasonably good response times.

But given the number of people who have been providing feedback
on this patch, and on the disk-read-latency patch, none of this
is going anywhere, and mine will be the only Linux machines which
don't suck.  (Takes ball, goes home).

-