Re: [PATCH] coroutine: cap per-thread local pool size

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Mar 20, 2024 at 09:35:39AM -0400, Stefan Hajnoczi wrote:
> On Tue, Mar 19, 2024 at 08:10:49PM +0000, Daniel P. Berrangé wrote:
> > On Tue, Mar 19, 2024 at 01:55:10PM -0400, Stefan Hajnoczi wrote:
> > > On Tue, Mar 19, 2024 at 01:43:32PM +0000, Daniel P. Berrangé wrote:
> > > > On Mon, Mar 18, 2024 at 02:34:29PM -0400, Stefan Hajnoczi wrote:
> > > > > diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> > > > > index 5fd2dbaf8b..2790959eaf 100644
> > > > > --- a/util/qemu-coroutine.c
> > > > > +++ b/util/qemu-coroutine.c
> > > > 
> > > > > +static unsigned int get_global_pool_hard_max_size(void)
> > > > > +{
> > > > > +#ifdef __linux__
> > > > > +    g_autofree char *contents = NULL;
> > > > > +    int max_map_count;
> > > > > +
> > > > > +    /*
> > > > > +     * Linux processes can have up to max_map_count virtual memory areas
> > > > > +     * (VMAs). mmap(2), mprotect(2), etc fail with ENOMEM beyond this limit. We
> > > > > +     * must limit the coroutine pool to a safe size to avoid running out of
> > > > > +     * VMAs.
> > > > > +     */
> > > > > +    if (g_file_get_contents("/proc/sys/vm/max_map_count", &contents, NULL,
> > > > > +                            NULL) &&
> > > > > +        qemu_strtoi(contents, NULL, 10, &max_map_count) == 0) {
> > > > > +        /*
> > > > > +         * This is a conservative upper bound that avoids exceeding
> > > > > +         * max_map_count. Leave half for non-coroutine users like library
> > > > > +         * dependencies, vhost-user, etc. Each coroutine takes up 2 VMAs so
> > > > > +         * halve the amount again.
> > 
> > Leaving half for loaded libraries, etc is quite conservative
> > if max_map_count is the small-ish 64k default.
> > 
> > That reservation could perhaps a fixed number like 5,000 ?
> 
> While I don't want QEMU to abort, once this heuristic is in the code it
> will be scary to make it more optimistic and we may never change it. So
> now is the best time to try 5,000.
> 
> I'll send a follow-up patch that reserves 5,000 mappings. If that turns
> out to be too optimistic we can increase the reservation.

BTW, I suggested 5,000, because I looked at a few QEM processes I have
running on Fedora and saw just under 1,000 lines in /proc/$PID/maps,
of which only a subset is library mappings. So multiplying that x5 felt
like a fairly generous overhead for more complex build configurations.

With regards,
Daniel
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