Subject: Re: [PATCH net-next 2/4] tools: ynl: allow setting recv() size

From: Jakub Kicinski <[email-masked]>
Date: Mon, 4 Mar 2024 06:58:23 -0800

On Mon, 4 Mar 2024 13:38:51 +0000 Donald Hunter wrote:
> >  class YnlFamily(SpecFamily):
> > -    def __init__(self, def_path, schema=None, process_unknown=False):
> > +    def __init__(self, def_path, schema=None, process_unknown=False,
> > +                 recv_size=131072):  
> 
> An aside: what is the reason for choosing a 128k receive buffer? If I
> remember correctly, netlink messages are capped at 32k.

Attributes, not messages, right? But large messages are relatively
rare, this is to make dump use fewer syscalls. Dump can give us multiple
message on each recv().

> >          super().__init__(def_path, schema)
> >
> >          self.include_raw = False
> > @@ -423,6 +428,16 @@ genl_family_name_to_id = None
> >          self.async_msg_ids = set()
> >          self.async_msg_queue = []
> >
> > +        # Note that netlink will use conservative (min) message size for
> > +        # the first dump recv() on the socket, our setting will only matter  
> 
> I'm curious, why does it behave like this?

Dump is initiated inside a send() system call, so that we can
validate arguments and return any init errors directly.
That means we don't know what buf size will be used by subsequent
recv()s when we produce the first message :(