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Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading prepa
From: |
Vivek Goyal |
Subject: |
Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3 |
Date: |
Thu, 8 Aug 2019 08:53:20 -0400 |
User-agent: |
Mutt/1.12.0 (2019-05-25) |
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.
>
> > 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
- Re: [Qemu-devel] [PATCH 3/4] virtiofsd: fix lo_destroy() resource leaks, (continued)
- [Qemu-devel] [PATCH 4/4] virtiofsd: add --thread-pool-size=NUM option, Stefan Hajnoczi, 2019/08/01
- Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3, piaojun, 2019/08/04
- Re: [Qemu-devel] [PATCH 0/4] virtiofsd: multithreading preparation part 3, Stefan Hajnoczi, 2019/08/07
- Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3, Vivek Goyal, 2019/08/07
- Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3, Stefan Hajnoczi, 2019/08/09
- Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3, Liu Bo, 2019/08/10
- Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3, piaojun, 2019/08/10
- Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3, Stefan Hajnoczi, 2019/08/12
- Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3, piaojun, 2019/08/12
- Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3, Dr. David Alan Gilbert, 2019/08/12
Re: [Qemu-devel] [Virtio-fs] [PATCH 0/4] virtiofsd: multithreading preparation part 3, piaojun, 2019/08/08