Re: 2.5.59-mm5

Nick Piggin
2003-01-24T15:55:40Z

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