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

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Date:	Thu, 21 May 2009 09:38:47 -0700
From:	Caitlin Bestler <caitlin.bestler@...il.com>
To:	Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:	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

> +
> +       while (datagrams < vlen) {
> +               err = __sys_recvmsg(sock, (struct msghdr __user *)entry, flags);
> +               if (err < 0)
> +                       goto out_put;
> +               err = __put_user(err, &entry->msg_len);
> +               if (err)
> +                       goto out_put;
> +               ++entry;
> +               ++datagrams;
> +       }
>  out_put:
>        fput_light(sock->file, fput_needed);
>  out:
> +       /*
> +        * We may return less entries than requested (vlen) if the
> +        * sock is non block and there aren't enough datagrams.
> +        */
> +       if (err == 0 || (err == -EAGAIN && (flags & MSG_DONTWAIT)))
> +               return datagrams;
>        return err;
>  }
>

There is an assumption here that unless MSG_DONTWAIT is set, or there
is an error,
that the caller will be willing to wait indefinitely for N messages to
show up -- and that
it is never worth waking up the caller earlier with less than N messages.

I think an application would more typically want to wait at most m
msecs after the first
message is received to see if any other messages can be delivered at
the same time.
A busy server could simply use DONTWAIT in a polling loop every cycle,
but it would be
nice to be able to wait indefinitely for *any* of your clients to send
you a message.

Further, with some sockets there are some messages that are more equal
than others.
Although valid messages, with no errors, they should be delivered to
user-mode immediately.
The example that leaps to my mind immediately are SCTP Events,
particularly with one-to-many
sockets. You could be waiting for N messages, knowing that a specific
peer has been asked to
send N messages. The 2nd message could be an SCTP event announcing
that the specific
association has been torn down (and hence the remaining messages will
not be arriving).
Waiting for a different association to send enough messages to
complete the request will
not provide very prompt service.
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