Subject: Re: [RFC 1/2] net: Introduce recvmmsg socket syscall

From: Steven Whitehouse <[email-masked]>
Date: Thu, 21 May 2009 17:33:35 +0100

Hi,

On Thu, 2009-05-21 at 13:27 -0300, Arnaldo Carvalho de Melo wrote:
> Em Thu, May 21, 2009 at 08:10:00PM +0400, Evgeniy Polyakov escreveu:
> > Hi Arnaldo.
> > 
> > On Wed, May 20, 2009 at 08:06:52PM -0300, Arnaldo Carvalho de Melo ([email-masked]) wrote:
> > > Meaning receive multiple messages, reducing the number of syscalls and
> > > net stack entry/exit operations.
> > > 
> > > Next patches will introduce mechanisms where protocols that want to
> > > optimize this operation will provide an unlocked_recvmsg operation.
> >  
> > What's the difference from the single msg with multiple iovecs?
> 
> recvmsg consumes just one skb, a datagram, truncating if it has more
> bytes than asked and giving less bytes than asked for if the skb being
> consumer is smaller than requested.
> 
> WRT iovec, it gets this skb/datagram and goes on filling iovec entry by
> entry, till it exhausts the skb.
> 
> The usecase here is: UDP socket has multiple skbs in its receive queue,
> so application will make several syscalls to get those skbs while we
> could return multiple datagrams in just one syscall + fd lookup + LSM
> validation + lock_sock + release_sock.
> 
Its not just UDP/SOCK_DGRAM either. SOCK_SEQPACKET has to return after
each message because it is not allowed to (nor can it, given the api)
return more than one message in a single call. So with small messages
that can add up to a lot of calls. This way the MSG_EOR flags can be
preserved in the correct places,

Steve.