On Thu, Aug 08, 2019 at 08:53:20AM -0400, Vivek Goyal wrote: > On Thu, Aug 08, 2019 at 10:53:16AM +0100, Dr. David Alan Gilbert wrote: > > * Stefan Hajnoczi (stefa...@redhat.com) wrote: > > > On Wed, Aug 07, 2019 at 04:57:15PM -0400, Vivek Goyal wrote: > > > > Kernel also serializes MAP/UNMAP on one inode. So you will need to run > > > > multiple jobs operating on different inodes to see parallel MAP/UNMAP > > > > (atleast from kernel's point of view). > > > > > > Okay, there is still room to experiment with how MAP and UNMAP are > > > handled by virtiofsd and QEMU even if the host kernel ultimately becomes > > > the bottleneck. > > > > > > One possible optimization is to eliminate REMOVEMAPPING requests when > > > the guest driver knows a SETUPMAPPING will follow immediately. I see > > > the following request pattern in a fio randread iodepth=64 job: > > > > > > unique: 995348, opcode: SETUPMAPPING (48), nodeid: 135, insize: 80, > > > pid: 1351 > > > lo_setupmapping(ino=135, fi=0x(nil), foffset=3860856832, len=2097152, > > > moffset=859832320, flags=0) > > > unique: 995348, success, outsize: 16 > > > unique: 995350, opcode: REMOVEMAPPING (49), nodeid: 135, insize: 60, > > > pid: 12 > > > unique: 995350, success, outsize: 16 > > > unique: 995352, opcode: SETUPMAPPING (48), nodeid: 135, insize: 80, > > > pid: 1351 > > > lo_setupmapping(ino=135, fi=0x(nil), foffset=16777216, len=2097152, > > > moffset=861929472, flags=0) > > > unique: 995352, success, outsize: 16 > > > unique: 995354, opcode: REMOVEMAPPING (49), nodeid: 135, insize: 60, > > > pid: 12 > > > unique: 995354, success, outsize: 16 > > > virtio_send_msg: elem 9: with 1 in desc of length 16 > > > unique: 995356, opcode: SETUPMAPPING (48), nodeid: 135, insize: 80, > > > pid: 1351 > > > lo_setupmapping(ino=135, fi=0x(nil), foffset=383778816, len=2097152, > > > moffset=864026624, flags=0) > > > unique: 995356, success, outsize: 16 > > > unique: 995358, opcode: REMOVEMAPPING (49), nodeid: 135, insize: 60, > > > pid: 12 > > > > > > The REMOVEMAPPING requests are unnecessary since we can map over the top > > > of the old mapping instead of taking the extra step of removing it > > > first. > > > > Yep, those should go - I think Vivek likes to keep them for testing > > since they make things fail more completely if there's a screwup. > > I like to keep them because otherwise they keep the resources busy > on host. If DAX range is being used immediately, then this optimization > makes more sense. I will keep this in mind. >
Other than the resource not being released, do you think there'll be any stale data problem if we don't do removemapping at all, neither background reclaim nor inline reclaim? (truncate/punch_hole/evict_inode still needs to remove mapping though) thanks, -liubo > > > > > Some more questions to consider for DAX performance optimization: > > > > > > 1. Is FUSE_READ/FUSE_WRITE more efficient than DAX for some I/O patterns? > > > > Probably for cases where the data is only accessed once, and you can't > > preemptively map. > > Another variant on (1) is whether we could do read/writes while the mmap > > is happening to absorb the latency. > > For small random I/O, dax might not be very effective. Overhead of > setting up mapping and tearing it down is significant. > > Vivek > > _______________________________________________ > Virtio-fs mailing list > virtio...@redhat.com > https://www.redhat.com/mailman/listinfo/virtio-fs