Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3

[Liu Bo]

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 (address@hidden) 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
> 
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