On Thu 14-07-11 16:08:24, Jeff Moyer wrote: > Jan Kara <jack@xxxxxxx> writes: > > > On Thu 14-07-11 12:30:32, Jeff Moyer wrote: > >> Tao Ma <tm@xxxxxx> 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... > > Sorry, I should have been more specific. As Vivek mentioned, we're > talking about older kernels (pre the blk plugging series). So, the > question is, if journal_commit_transaction is called with > t_synchronous_commit not set, will the underlying device ever be > unplugged by the journal code? My guess is there's no explicit unplug, > so it's not correct to replace a WRITE_SYNC with a WRITE_SYNC_PLUG. There are no explicit unplugs in journalling code. But checking the code in 2.6.37, I still see wait_on_buffer() calls sync_bh() which calls blk_run_address_space() which ends up calling bdi->unplug_io_fn() so I would say unplug is called anyway. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
