Re: multifd/tcp/zlib intermittent abort (was: Re: [PULL 00/18] migration queue)

[Peter Maydell]

On Tue, 15 Mar 2022 at 14:39, Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Mon, 14 Mar 2022 at 19:44, Peter Maydell <peter.maydell@linaro.org> wrote:
> > On Mon, 14 Mar 2022 at 18:58, Peter Maydell <peter.maydell@linaro.org> 
> > wrote:
> > > I just hit the abort case, narrowing it down to the
> > > /i386/migration/multifd/tcp/zlib case, which can hit this without
> > > any other tests being run:
> >
> > > This test seems to fail fairly frequently. I'll try a bisect...
> >
> > On this s390 machine, this test has been intermittent since
> > it was first added in commit 7ec2c2b3c1 ("multifd: Add zlib compression
> > multifd support") in 2019.
>
> I have tried (on current master) runs of various of the other
> migration tests, and:
>  * /i386/migration/multifd/tcp/zstd completed 1170 iterations without
>    failing
>  * /i386/migration/precopy/tcp completed 4669 iterations without
>    failing
>  * /i386/migration/multifd/tcp/zlib fails usually within the first
>    10 iterations (the most I ever saw it manage was 32)
>
> So whatever this is, it seems like it might be specific to the
> zlib code somehow ?

Maybe we're running into this bug
https://bugs.launchpad.net/ubuntu/+source/zlib/+bug/1961427
("zlib: compressBound() returns an incorrect result on z15") ?

That bug report claims it doesn't affect focal, though, which
is what we're running on this box (specifically, the zlib1g
package is version 1:1.2.11.dfsg-2ubuntu1.2).

A run with DFLTCC=0 has made it past 60 iterations so far, which
suggests that that does serve as a workaround for the bug.

thanks
-- PMM