Re: [PATCH v4 00/12] remove msize limit in virtio transport

[Nikolay Kichukov <nikolay@oldum.net>]

Thanks Christian,
It works, sorry for overlooking the 'known limitations' in the first
place. When do we expect these patches to be merged upstream?

Cheers,
-N


On Sat, 2022-01-22 at 14:34 +0100, Christian Schoenebeck wrote:
> On Donnerstag, 20. Januar 2022 23:43:46 CET Nikolay Kichukov wrote:
> > Thanks for the patches. I've applied them on top of 5.16.2 kernel
> > and it
> > works for msize=1048576. Performance-wise, same throughput as the
> > previous patches, basically limiting factor is the backend block
> > storage.
> 
> Depends on how you were testing exactly. I assume you just booted a
> guest and 
> then mounted a humble 9p directory in guest to perform some isolated
> I/O 
> throughput tests on a single file. In this test scenario yes, you
> would not 
> see much of a difference between v3 vs. v4 of this series.
> 
> However in my tests I went much further than that by running an entire
> guest 
> on top of 9p as its root filesystem:
> https://wiki.qemu.org/Documentation/9p_root_fs
> With this 9p rootfs setup you get a completely different picture. For
> instance 
> you'll notice with v3 that guest boot time *increases* with rising
> msize, 
> whereas with v4 it shrinks. And also when you benchmark throughput on
> a file 
> in this 9p rootfs setup with v3 you get worse results than with v4,
> sometimes 
> with v3 even worse than without patches at all. With v4 applied though
> it 
> clearly outperforms any other kernel version in all aspects.
> 
> I highly recommend this 9p rootfs setup as a heterogenous 9p test
> environment, 
> as it is a very good real world test scenario for all kinds of
> aspects.
> 
> > However, when I mount with msize=4194304, the system locks up upon
> > first
> > try to traverse the directory structure, ie 'ls'. Only solution is
> > to
> > 'poweroff' the guest. Nothing in the logs.
> 
> I've described this in detail in the cover letter under "KNOWN
> LIMITATIONS" 
> already. Use max. msize 4186112.
> 
> > Qemu 6.0.0 on the host has the following patches:
> > 
> > 01-fix-wrong-io-block-size-Rgetattr.patch
> > 02-dedupe-iounit-code.patch
> > 03-9pfs-simplify-blksize_to_iounit.patch
> 
> I recommend just using QEMU 6.2. It is not worth to patch that old
> QEMU 
> version. E.g. you would have a lousy readdir performance with that
> QEMU 
> version and what not.
> 
> You don't need to install QEMU. You can directly run it from the build
> directory.
> 
> > The kernel patches were applied on the guest kernel only.
> > 
> > I've generated them with the following command:
> > git diff
> > 783ba37c1566dd715b9a67d437efa3b77e3cd1a7^..8c305df4646b65218978fc647
> > 4aa0f5f
> > 29b216a0 > /tmp/kernel-5.16-9p-virtio-drop-msize-cap.patch
> > 
> > The host runs 5.15.4 kernel.
> 
> Host kernel version currently does not matter for this series.
> 
> Best regards,
> Christian Schoenebeck
> 
>