On Tue, 17 Aug 2021 at 15:08, JeffleXu <jefflexu@xxxxxxxxxxxxxxxxx> wrote: > > > > On 8/17/21 6:09 PM, Miklos Szeredi wrote: > > On Tue, 17 Aug 2021 at 11:32, Dr. David Alan Gilbert > > <dgilbert@xxxxxxxxxx> wrote: > >> > >> * Miklos Szeredi (miklos@xxxxxxxxxx) wrote: > >>> On Tue, 17 Aug 2021 at 04:22, Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx> wrote: > >>>> > >>>> This patchset adds support of per-file DAX for virtiofs, which is > >>>> inspired by Ira Weiny's work on ext4[1] and xfs[2]. > >>> > >>> Can you please explain the background of this change in detail? > >>> > >>> Why would an admin want to enable DAX for a particular virtiofs file > >>> and not for others? > >> > >> Where we're contending on virtiofs dax cache size it makes a lot of > >> sense; it's quite expensive for us to map something into the cache > >> (especially if we push something else out), so selectively DAXing files > >> that are expected to be hot could help reduce cache churn. > > > > If this is a performance issue, it should be fixed in a way that > > doesn't require hand tuning like you suggest, I think. > > > > I'm not sure what the ext4/xfs case for per-file DAX is. Maybe that > > can help understand the virtiofs case as well. > > > > Some hints why ext4/xfs support per-file DAX can be found [1] and [2]. > > "Boaz Harrosh wondered why someone might want to turn DAX off for a > persistent memory device. Hellwig said that the performance "could > suck"; Williams noted that the page cache could be useful for some > applications as well. Jan Kara pointed out that reads from persistent > memory are close to DRAM speed, but that writes are not; the page cache > could be helpful for frequent writes. Applications need to change to > fully take advantage of DAX, Williams said; part of the promise of > adding a flag is that users can do DAX on smaller granularities than a > full filesystem." > > In summary, page cache is preferable in some cases, and thus more fine > grained way of DAX control is needed. Hmm, okay, very frequent overwrites could be problematic for directly mapped nvram. > > As for virtiofs, Dr. David Alan Gilbert has mentioned that various files > may compete for limited DAX window resource. > > Besides, supporting DAX for small files can be expensive. Small files > can consume DAX window resource rapidly, and if small files are accessed > only once, the cost of mmap/munmap on host can not be ignored. That's a good point. Maybe we should disable DAX for file sizes much smaller than the chunk size? Thanks, Miklos
