Re: 2.5.59-mm5

Oliver Xymoron
2003-01-24T15:47:30Z

On Fri, Jan 24, 2003 at 03:50:17AM -0800, Andrew Morton wrote:
> Alex Tomas <bzzz@tmi.comex.ru> wrote:
> >
> > >>>>> Andrew Morton (AM) writes:
> >
> > AM> But writes are completely different. There is no dependency
> > AM> between them and at any point in time we know where on-disk a lot
> > AM> of writes will be placed. We don't know that for reads, which is
> > AM> why we need to twiddle thumbs until the application or filesystem
> > AM> makes up its mind.
> >
> >
> > it's significant that application doesn't want to wait read completion
> > long and doesn't wait for write completion in most cases.
>
> That's correct. Reads are usually synchronous and writes are rarely
> synchronous.
>
> The most common place where the kernel forces a user process to wait on
> completion of a write is actually in unlink (truncate, really). Because
> truncate must wait for in-progress I/O to complete before allowing the
> filesystem to free (and potentially reuse) the affected blocks.
>
> If there's a lot of writeout happening then truncate can take _ages_. Hence
> this patch:

An alternate approach might be to change the way the scheduler splits
things. That is, rather than marking I/O read vs write and scheduling
based on that, add a flag bit to mark them all sync vs async since
that's the distinction we actually care about. The normal paths can
all do read+sync and write+async, but you can now do things like
marking your truncate writes sync and readahead async.

And dependent/nondependent or stalling/nonstalling might be a clearer
terminology.

--
"Love the dolphins," she advised him. "Write by W.A.S.T.E.."