Messages in this thread | | | Date | Mon, 4 May 2015 20:54:37 +0800 | Subject | Re: [PATCH v6] block: loop: avoiding too many pending per work I/O | From | Ming Lei <> |
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On Sun, May 3, 2015 at 9:52 AM, Tejun Heo <tj@kernel.org> wrote: > Hello, > > On Sat, May 02, 2015 at 10:56:20PM +0800, Ming Lei wrote: >> > Maybe just cap max_active to NR_OF_LOOP_DEVS * 16 or sth? But idk, >> >> It might not work because there are nested loop devices like fedora live CD, and >> in theory the max_active should have been set as loop's queue depth * >> nr_loop, otherwise there may be possibility of hanging. >> >> So this patch is introduced. > > If loop devices can be stacked, regardless of what you do with > nr_active, it may deadlock. There needs to be a rescuer per each > nesting level (or just one per device). This means that the current > code is broken.
Yes.
>> > how many concurrent workers are we talking about and why are we >> > capping per-queue concurrency from worker pool side instead of command >> > tag side? >> >> I think there should be performance advantage to make queue depth a bit more >> because it can help to make queue pipeline as full. Also queue depth often >> means how many requests the hardware can queue, and it is a bit different >> with per-queue concurrency. > > I'm not really following. Can you please elaborate?
In case of loop-mq, a bigger queue_depth often has better performance when doing read/write page cache in sequential read/write because they are very quick and better to run them as a batch in one time work function, but simply deceasing queue depth may hurt performance for this case.
Thanks, Ming Lei
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