Messages in this thread | | | Date | Fri, 29 Oct 2010 13:46:15 -0700 (PDT) | From | Davide Libenzi <> | Subject | Re: [PATCH 0/1] RFC: poll/select performance on datagram sockets |
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On Fri, 29 Oct 2010, Davide Libenzi wrote:
> On Fri, 29 Oct 2010, Eric Dumazet wrote: > > > 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. > > Yeah, epoll does check for event hints coming with the callback wakeup, > and avoid waking up epoll_wait() waiters, for non matching events.
Also, why not using the existing wait->key instead of adding a poll2()?
- Davide
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