On Wed, Jun 13, 2012 at 10:48:40PM +0800, Fengguang Wu wrote: > Hi Jeff, > > On Wed, Jun 13, 2012 at 10:27:50AM -0400, Jeff Moyer wrote: > > Wanpeng Li <liwp.linux@xxxxxxxxx> writes: > > > > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > > > index f2d0109..df879ee 100644 > > > --- a/fs/fs-writeback.c > > > +++ b/fs/fs-writeback.c > > > @@ -1311,7 +1311,11 @@ void writeback_inodes_sb_nr(struct super_block *sb, > > > > > > WARN_ON(!rwsem_is_locked(&sb->s_umount)); > > > bdi_queue_work(sb->s_bdi, &work); > > > - wait_for_completion(&done); > > > + if (sysctl_hung_task_timeout_secs) > > > + while (!wait_for_completion_timeout(&done, HZ/2)) > > > + ; > > > + else > > > + wait_for_completion(&done); > > > } > > > EXPORT_SYMBOL(writeback_inodes_sb_nr); > > > > Is it really expected that writeback_inodes_sb_nr will routinely queue > > up more than 2 seconds worth of I/O (Yes, I understand that it isn't the > > only entity issuing I/O)? > > Yes, in the case of syncing the whole superblock. > Basically sync() does its job in two steps: > > for all sb: > writeback_inodes_sb_nr() # WB_SYNC_NONE > sync_inodes_sb() # WB_SYNC_ALL > > > For devices that are really slow, it may make > > more sense to tune the system so that you don't have too much writeback > > I/O submitted at once. Dropping nr_requests for the given queue should > > fix this situation, I would think. Note that this wait_for_completion() waits for the flusher to move all dirty pages into writeback pages, which in turn will block in get_request_wait(). So writeback_inodes_sb_nr() could in theory finish quickly if the nr_requests is a huge value. But then sync_inodes_sb() will come and sleep (longer) on each of the writeback pages.. Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
