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. > > > 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
