On Thu 14-07-11 12:30:32, Jeff Moyer wrote:
> Tao Ma <[email protected]> writes:
> >> - WRITE_SYNC_PLUG will plug the queue and expects explicity unplug. Who
> >> is doing unplug in this case?
> > See the comments I removed, "we rely on sync_buffer() doing the unplug
> > for us". I removed them cause we all use pluged write now.
>
> Your logic is upside-down. The code currently only uses the _PLUG
> variant when t_synchronous_commit is set, meaning somebody *will* call
> sync_buffer. Simply setting WRITE_SYNC_PLUG doens't mean the upper
> layer is going to issue the unplug. Of course, I'm not 100% sure of the
> journaling process, so it may very well be that there always is an
> unplug. Can Jan or someone comment on that? Anyway, you could test
> this theory by seeing if your kernel generates any timer unplugs in the
> blktrace output.
So I'm not expert in plugging code but from what I understand when we do
wait_on_buffer() (which calls io_schedule()) which will do
blk_flush_plug()), the queue will get unplugged and IO starts. And we wait
for all buffers we submit so we are guaranteed wait_on_buffer() will be
called...
Honza
--
Jan Kara <[email protected]>
SUSE Labs, CR
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