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

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Message-Id: <1242923615.29604.409.camel@localhost.localdomain>
Date:	Thu, 21 May 2009 17:33:35 +0100
From:	Steven Whitehouse <steve@...gwyn.com>
To:	Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:	Evgeniy Polyakov <zbr@...emap.net>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Chris Van Hoof <vanhoof@...hat.com>,
	Clark Williams <williams@...hat.com>
Subject: Re: [RFC 1/2] net: Introduce recvmmsg socket syscall

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 (acme@...hat.com) 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.


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