On Fri, 6 Dec 2019, Nikos Tsironis wrote: > On 11/22/19 8:55 PM, Joe Thornber wrote: > > On Fri, Nov 22, 2019 at 11:14:15AM +0800, JeffleXu wrote: > > > > > The first question is what's the purpose of data cell? In thin_bio_map(), > > > normal bio will be packed as a virtual cell and data cell. I can > > > understand > > > that virtual cell is used to prevent discard bio and non-discard bio > > > targeting the same block from being processed at the same time. I find it > > > was added in commit e8088073c9610af017fd47fddd104a2c3afb32e8 (dm thin: > > > fix race between simultaneous io and discards to same block), but I'm > > > still > > > confused about the use of data cell. > > > > As you are aware there are two address spaces for the locks. The 'virtual' > > one > > refers to cells in the logical address space of the thin devices, and the > > 'data' one > > refers to the underlying data device. There are certain conditions where we > > unfortunately need to hold both of these (eg, to prevent a data block being > > reprovisioned > > before an io to it has completed). > > > > > The second question is the impact of virtual cell and data cell on IO > > > performance. If $data_block_size is large for example 1G, in multithread > > > fio > > > test, most bio will be buffered in cell->bios list and then be processed > > > by > > > worker thread asynchronously, even when there's no discard bio. Thus the > > > original parallel IO is processed by worker thread serially now. As the > > > number of fio test threads increase, the single worker thread can easily > > > get > > > CPU 100%, and thus become the bottleneck of the performance since dm-thin > > > workqueue is ordered unbound. > > > > Yep, this is a big issue. Take a look at dm-bio-prison-v2.h, this is the > > new interface that we need to move dm-thin across to use (dm-cache already > > uses it). > > It allows concurrent holders of a cell (ie, read locks), so we'll be able to > > remap > > much more io without handing it off to a worker thread. Once this is done I > > want > > to add an extra field to cells that will cache the mapping, this way if you > > acquire a > > cell that is already held then you can avoid the expensive btree lookup. > > Together > > these changes should make a huge difference to the performance. > > > > If you've got some spare coding cycles I'd love some help with this ;) > > > > Hi Joe, > > I would be interested in helping you with this task. I can't make any > promises, but I believe I could probably spare some time to work on it. Hi Nikos, it would be great if you are able help with the dm-thin port to dm-bio-prison-v2. I'm glad to see you are interested in dm-thin performance too. These are the commits that implemented dm-bio-prison-v2 in dm-cache back in ~4.12, maybe it can give you a good start on what the conversion might look like: b29d4986d dm cache: significant rework to leverage dm-bio-prison-v2 Here's a related bugfix: d1260e2a3 dm cache: fix race condition in the writeback mode overwrite_bio optimisation -- Eric Wheeler > > Nikos > > > - Joe > > > > -- > > dm-devel mailing list > > dm-devel@xxxxxxxxxx > > https://www.redhat.com/mailman/listinfo/dm-devel > > > > -- > dm-devel mailing list > dm-devel@xxxxxxxxxx > https://www.redhat.com/mailman/listinfo/dm-devel > > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
