Re: [PATCH 2/2] cfq-iosched: RQ_NOIDLE enabled for SYNC_WORKLOAD

[Christoph Hellwig <hch@infradead.org>]

On Wed, Jul 07, 2010 at 11:46:31AM -0400, Vivek Goyal wrote:
> Whether to idle on WRITE_SYNC or not, I will leave it to Jens (I just
> don't know the answer to that question. :-)). But in general I want to
> get rid of idling as much as possible otherwise it becomes a serious
> bottleneck in any kind of performance testing on higher end storage.

After I've been thinking about this for a while I think the major
problems is that we use WRITE_SYNC for two very different I/O patterns.

One is synchronous data I/O (O_SYNC/O_DIRECT/fsync).  While this is a
high-level synchronous workload in the sense that someone waits for the
I/O to finish, the I/O can still be batched as we're doing relatively
large amounts of bios.

The other one is synchronous writeout of metadata or the journal.  Here
we typically wait on that single I/O we're just submitting (or at most a
handfull), and there is absolutely no point in idling.

We already have the REQ_NOIDLE flag to distinguish between the two, so
instead of second guessing we should actually make use of it.