Messages in this thread |  | | Date | Fri, 24 Jan 2003 09:56:26 -0600 | From | Oliver Xymoron <> | Subject | Re: 2.5.59-mm5 |
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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.
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