On Thu 24-09-09 16:33:42, Wu Fengguang wrote: > On Mon, Sep 14, 2009 at 07:17:21PM +0800, Jan Kara wrote: > > On Thu 10-09-09 17:49:10, Peter Zijlstra wrote: > > > On Wed, 2009-09-09 at 16:23 +0200, Jan Kara wrote: > > > > Well, what I imagined we could do is: > > > > Have a per-bdi variable 'pages_written' - that would reflect the amount of > > > > pages written to the bdi since boot (OK, we'd have to handle overflows but > > > > that's doable). > > > > > > > > There will be a per-bdi variable 'pages_waited'. When a thread should sleep > > > > in balance_dirty_pages() because we are over limits, it kicks writeback thread > > > > and does: > > > > to_wait = max(pages_waited, pages_written) + sync_dirty_pages() (or > > > > whatever number we decide) > > > > pages_waited = to_wait > > > > sleep until pages_written reaches to_wait or we drop below dirty limits. > > > > > > > > That will make sure each thread will sleep until writeback threads have done > > > > their duty for the writing thread. > > > > > > > > If we make sure sleeping threads are properly ordered on the wait queue, > > > > we could always wakeup just the first one and thus avoid the herding > > > > effect. When we drop below dirty limits, we would just wakeup the whole > > > > waitqueue. > > > > > > > > Does this sound reasonable? > > > > > > That seems to go wrong when there's multiple tasks waiting on the same > > > bdi, you'd count each page for 1/n its weight. > > > > > > Suppose pages_written = 1024, and 4 tasks block and compute their to > > > wait as pages_written + 256 = 1280, then we'd release all 4 of them > > > after 256 pages are written, instead of 4*256, which would be > > > pages_written = 2048. > > Well, there's some locking needed of course. The intent is to stack > > demands as they come. So in case pages_written = 1024, pages_waited = 1024 > > we would do: > > THREAD 1: > > > > spin_lock > > to_wait = 1024 + 256 > > pages_waited = 1280 > > spin_unlock > > > > THREAD 2: > > > > spin_lock > > to_wait = 1280 + 256 > > pages_waited = 1536 > > spin_unlock > > > > So weight of each page will be kept. The fact that second thread > > effectively waits until the first thread has its demand satisfied looks > > strange at the first sight but we don't do better currently and I think > > it's fine - if they were two writer threads, then soon the thread released > > first will queue behind the thread still waiting so long term the behavior > > should be fair. > > Yeah, FIFO queuing should be good enough. > > I'd like to propose one more data structure for evaluation :) > > - bdi->throttle_lock > - bdi->throttle_list pages to sync for each waiting task, taken from sync_writeback_pages() > - bdi->throttle_pages (counted down) pages to sync for the head task, shall be atomic_t > > In balance_dirty_pages(), it would do > > nr_to_sync = sync_writeback_pages() > if (list_empty(bdi->throttle_list)) # I'm the only task > bdi->throttle_pages = nr_to_sync > append nr_to_sync to bdi->throttle_list > kick off background writeback > wait > remove itself from bdi->throttle_list and wait list > set bdi->throttle_pages for new head task (or LONG_MAX) > > In __bdi_writeout_inc(), it would do > > if (--bdi->throttle_pages <= 0) > check and wake up head task Yeah, this would work as well. I don't see a big difference between my approach and this so if you get to implementing this, I'm happy :). > In wb_writeback(), it would do > > if (args->for_background && exiting) > wake up all throttled tasks > To prevent wake up too many tasks at the same time, it can relax the > background threshold a bit, so that __bdi_writeout_inc() become the > only wake up point in normal cases. > > if (args->for_background && !list_empty(bdi->throttle_list) && > over background_thresh - background_thresh / 32) > keep write pages; We want to wakeup tasks when we get below dirty_limit (either global or per-bdi). Not when we get below background threshold... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
