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

[Dr. David Alan Gilbert]

* 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.

> 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.

> 2. Can MAP/UNMAP be performed directly in QEMU via a separate virtqueue?

I think there's two things to solve here that I don't currently know the
answer to:
  2a) We'd need to get the fd to qemu for the thing to mmap;
      we might be able to cache the fd on the qemu side for existing
      mappings, so when asking for a new mapping for an existing file then
      it would already have the fd.

  2b) Running a device with a mix of queues inside QEMU and on
      vhost-user; I don't think we have anything with that mix
 
> 3. Can READ/WRITE be performed directly in QEMU via a separate virtqueue
>    to eliminate the bad address problem?

Are you thinking of doing all read/writes that way, or just the corner
cases? It doesn't seem worth it for the corner cases unless you're
finding them cropping up in real work loads.

> 4. Can OPEN+MAP be fused into a single request for small files, avoiding
>    the 2nd request?

Sounds possible.

> I'm not going to tackle DAX optimization myself right now but wanted to
> share these ideas.

One I was thinking about that feels easier than (2) was to change the
vhost slave protocol to be split transaction; it wouldn't do anything
for the latency but it would be able to do some in parallel if we can
get the kernel to feed it.

Dave

> Stefan



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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK