Messages in this thread | | | Date | Sat, 25 Jan 2003 03:04:49 +1100 | From | Nick Piggin <> | Subject | Re: 2.5.59-mm5 |
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Oliver Xymoron wrote:
>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. > That will be worth investigating to see if the complexity is worth it. I think from a disk point of view, we still want to split batches between reads and writes. Could be wrong.
Nick
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