Hi Vivek and Jeff, On 07/15/2011 05:38 AM, Jan Kara wrote: > 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. yeah, jbd2 works like what Jan described. And what's more, if you looked at the commit I mentioned(749ef9f8423), this commit just changed WRITE to WRITE_SYNC, so in the old times(before that commit is merged), WRITE has been used in jbd/2 for many years. It also doesn't do a explicit unplug, let the scheduler do the merge and reply on sync_bh to unplug the device. So change WRITE to WRITE_SYNC_PLUG is welcomed, but change WRITE to WRITE_SYNC is broken since it splits the sequential write to several i/o requests. Thanks Tao -- 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
