On Thu, 24 May 2012 13:17:09 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote: > On Thu, May 24, 2012 at 01:34:02PM +1000, NeilBrown wrote: > > On Wed, 23 May 2012 15:26:21 +0800 Shaohua Li <shli@xxxxxxxxxx> wrote: > > > > > In raid1, all write requests are dispatched in raid1d thread. In fast storage, > > > the raid1d thread is a bottleneck, because it dispatches request too slow. Also > > > raid1d thread migrates freely, which makes request completion cpu not match > > > with submission cpu even driver/block layer has such capability. This will > > > cause bad cache issue. > > > > > > If bitmap support is enabled, write requests can only be dispatched after dirty > > > bitmap is flushed out. After bitmap is flushed, how write requests are > > > dispatched doesn't impact correctness. A natural idea is to distribute request > > > dispatch to several threads. With this patch, requests are added to a percpu > > > list first. After bitmap is flushed, then the percpu list requests will > > > dispatched in a workqueue. In this way, above bottleneck is removed. > > > > > > In a 4k randwrite test with a 2 disks setup, below patch can provide 10% ~ 50% > > > performance improvements depending on numa binding. > > > > Those numbers are quite impressive so there is certainly room for improvement > > here. I'm not sure that I'm entirely comfortable with the approach though. > > > > Passing the request to a per-cpu thread does make sense, but a generic > > per-cpu thread feels dangerous as we don't know what else might be queued to > > that thread and because of the potential for deadlocks between memory > > allocation and generic_make_request (as mentioned in previous email) I find > > it hard to convince myself that this approach is entirely safe. > > No problem, we can use a separate workqueue. A separate set of per-cpu workqueues for each md array doesn't sound like a good idea to me - if we can possibly avoid it. > > > I wonder if we might take a very different approach and try to do everything > > in the one process. i.e. don't hand tasks off to other threads at all - at > > least in the common case. > > So we could change plugger_unplug (in md.c) so that: > > > > - if current->bio_list is NULL, (meaning all requests have been submitted > > and there is no risk of deadlock) we call bitmap_unplug and then submit > > all the queued writes. > > - if current->bio_list is not NULL, then we just wakeup the md thread to > > do the work. > > The current->bio_list check does make sense. I'm going to do the check for the > 1 and 3 patches. > > But I believe we can't call generic_make_request in unplug. For example, > schedule()->unplug->generic_make_request->get_request_wait()->schedule(). At My ideas was to only submit the queued writes from a normal call to blk_finish_plug. Calls that come from schedule would use the current approach of signalling the md thread to take over. There must be a way to tell the difference - maybe just a new flag to the unplug callback. NeilBrown > least this will cause some request not dispatch. And last time I did similar > experiment for raid0 (request is added to a per-disk plug_cb list, not directly > dispatch) to reduce lock contention, I found nasty oops. > > Thanks, > Shaohua
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