On Fri, Jan 19, 2018 at 09:23:35AM -0700, Jens Axboe wrote: > On 1/19/18 9:13 AM, Mike Snitzer wrote: > > On Fri, Jan 19 2018 at 10:48am -0500, > > Jens Axboe <axboe@xxxxxxxxx> wrote: > > > >> On 1/19/18 8:40 AM, Ming Lei wrote: > >>>>>> Where does the dm STS_RESOURCE error usually come from - what's exact > >>>>>> resource are we running out of? > >>>>> > >>>>> It is from blk_get_request(underlying queue), see > >>>>> multipath_clone_and_map(). > >>>> > >>>> That's what I thought. So for a low queue depth underlying queue, it's > >>>> quite possible that this situation can happen. Two potential solutions > >>>> I see: > >>>> > >>>> 1) As described earlier in this thread, having a mechanism for being > >>>> notified when the scarce resource becomes available. It would not > >>>> be hard to tap into the existing sbitmap wait queue for that. > >>>> > >>>> 2) Have dm set BLK_MQ_F_BLOCKING and just sleep on the resource > >>>> allocation. I haven't read the dm code to know if this is a > >>>> possibility or not. > > > > Right, #2 is _not_ the way forward. Historically request-based DM used > > its own mempool for requests, this was to be able to have some measure > > of control and resiliency in the face of low memory conditions that > > might be affecting the broader system. > > > > Then Christoph switched over to adding per-request-data; which ushered > > in the use of blk_get_request using ATOMIC allocations. I like the > > result of that line of development. But taking the next step of setting > > BLK_MQ_F_BLOCKING is highly unfortunate (especially in that this > > dm-mpath.c code is common to old .request_fn and blk-mq, at least the > > call to blk_get_request is). Ultimately dm-mpath like to avoid blocking > > for a request because for this dm-mpath device we have multiple queues > > to allocate from if need be (provided we have an active-active storage > > network topology). > > If you can go to multiple devices, obviously it should not block on a > single device. That's only true for the case where you can only go to > one device, blocking at that point would probably be fine. Or if all > your paths are busy, then blocking would also be OK. Introducing one extra block point will hurt AIO performance, in which there is usually much less jobs/processes to submit IO. -- Ming -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
