Re: [PATCH] multifd: Set a higher "backlog" default value for listen()

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

On Thu, May 18, 2023 at 09:13:58AM +0000, Wang, Wei W wrote:
> On Thursday, May 18, 2023 4:52 PM, Wang, Lei4 wrote:
> > When destination VM is launched, the "backlog" parameter for listen() is set
> > to 1 as default in socket_start_incoming_migration_internal(), which will
> > lead to socket connection error (the queue of pending connections is full)
> > when "multifd" and "multifd-channels" are set later on and a high number of
> > channels are used. Set it to a hard-coded higher default value 512 to fix this
> > issue.
> > 
> > Reported-by: Wei Wang <wei.w.wang@intel.com>
> > Signed-off-by: Lei Wang <lei4.wang@intel.com>
> > ---
> >  migration/socket.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/migration/socket.c b/migration/socket.c index
> > 1b6f5baefb..b43a66ef7e 100644
> > --- a/migration/socket.c
> > +++ b/migration/socket.c
> > @@ -179,7 +179,7 @@
> > socket_start_incoming_migration_internal(SocketAddress *saddr,
> >      QIONetListener *listener = qio_net_listener_new();
> >      MigrationIncomingState *mis = migration_incoming_get_current();
> >      size_t i;
> > -    int num = 1;
> > +    int num = 512;
> > 
> 
> Probably we need a macro for it, e.g.
> #define MIGRATION_CHANNEL_MAX  512
> 
> Also, I think below lines could be removed, as using a larger value of num (i.e. 512)
> doesn't seem to consume more resources anywhere:
> -    if (migrate_use_multifd()) {
> -        num = migrate_multifd_channels();
> -    } else if (migrate_postcopy_preempt()) {
> -        num = RAM_CHANNEL_MAX;
> -    }

Given that this code already exists, why is it not already sufficient ?

The commit description is saying we're setting backlog == 1 wit
multifd, but this later code is setting it to match the multfd
channels.  Why isn't that enough ?

With regards,
Daniel
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