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

[Juan Quintela <quintela@redhat.com>]

"Wang, Wei W" <wei.w.wang@intel.com> 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:

Could you confirm that?

> -    if (migrate_use_multifd()) {
> -        num = migrate_multifd_channels();
> -    } else if (migrate_postcopy_preempt()) {
> -        num = RAM_CHANNEL_MAX;
> -    }

Agreed that in this case we should drop this bit.

But on the other hand, if it does'nt consume more resources, why isn't
the kernel just ignoring the value passed to listen an just use a big
number?

Later, Juan.