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Date: Sat, 30 Oct 2010 14:53:37 +0200 From: Eric Dumazet <eric.dumazet@...il.com> To: Alban Crequy <alban.crequy@...labora.co.uk> Cc: "David S. Miller" <davem@...emloft.net>, Stephen Hemminger <shemminger@...tta.com>, Cyrill Gorcunov <gorcunov@...nvz.org>, Alexey Dobriyan <adobriyan@...il.com>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Pauli Nieminen <pauli.nieminen@...labora.co.uk>, Rainer Weikusat <rweikusat@...gmbh.com>, Davide Libenzi <davidel@...ilserver.org> Subject: Re: [PATCH 0/1] RFC: poll/select performance on datagram sockets Le samedi 30 octobre 2010 à 12:34 +0100, Alban Crequy a écrit : > Le Fri, 29 Oct 2010 21:27:11 +0200, > Eric Dumazet <eric.dumazet@...il.com> a écrit : > > > Le vendredi 29 octobre 2010 à 19:18 +0100, Alban Crequy a écrit : > > > Hi, > > > > > > When a process calls the poll or select, the kernel calls (struct > > > file_operations)->poll on every file descriptor and returns a mask > > > of events which are ready. If the process is only interested by > > > POLLIN events, the mask is still computed for POLLOUT and it can be > > > expensive. For example, on Unix datagram sockets, a process running > > > poll() with POLLIN will wakes-up when the remote end call read(). > > > This is a performance regression introduced when fixing another bug > > > by 3c73419c09a5ef73d56472dbfdade9e311496e9b and > > > ec0d215f9420564fc8286dcf93d2d068bb53a07e. > > > > > > The attached program illustrates the problem. It compares the > > > performance of sending/receiving data on an Unix datagram socket and > > > select(). When the datagram sockets are not connected, the > > > performance problem is not triggered, but when they are connected > > > it becomes a lot slower. On my computer, I have the following time: > > > > > > Connected datagram sockets: >4 seconds > > > Non-connected datagram sockets: <1 second > > > > > > The patch attached in the next email fixes the performance problem: > > > it becomes <1 second for both cases. I am not suggesting the patch > > > for inclusion; I would like to change the prototype of (struct > > > file_operations)->poll instead of adding ->poll2. But there is a > > > lot of poll functions to change (grep tells me 337 functions). > > > > > > Any opinions? > > > > My opinion would be to use epoll() for this kind of workload. > > I found a problem with epoll() with the following program. When there > is several datagram sockets connected to the same server and the > receiving queue is full, epoll(EPOLLOUT) wakes up only the emitter who > has its skb removed from the queue, and not all the emitters. It is > because sock_wfree() runs sk->sk_write_space() only for one emitter. > I dont think this is the reason. sock_wfree() really is good here, since it copes with one socket (the one that sent the message) Problem is the peer_wait, that epoll doesnt seem to be plugged into. Bug is in unix_dgram_poll() It calls sock_poll_wait( ... &unix_sk(other)->peer_wait,) only if socket is 'writable'. Its a clear bug Try this patch please ? diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c index 0ebc777..315716c 100644 --- a/net/unix/af_unix.c +++ b/net/unix/af_unix.c @@ -2092,7 +2092,7 @@ static unsigned int unix_dgram_poll(struct file *file, struct socket *sock, /* writable? */ writable = unix_writable(sk); - if (writable) { + if (1 /*writable*/) { other = unix_peer_get(sk); if (other) { if (unix_peer(other) != sk) { -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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