bug in big pipe code causing performance problems

From: Pierre Beyssac (pb_at_fasterix.frmug.org)
Date: 07/30/03

  • Next message: Juli Mallett: "Re: make -U" 
    Date: Wed, 30 Jul 2003 23:32:49 +0200
To: freebsd-current@freebsd.org

    Hello,

    If no one objects to it, I'd like to commit the following ASAP. It
    fixes an obvious bug in the big pipe code, a discrepancy between
    how free space is calculated in write vs poll. The bug affects
    stable as well.

    The bug's implications are less obvious: it make write(2) on a
    non-blocking pipe return EAGAIN when poll(2) & select(2) return the
    descriptor as ready for write. This in turns causes the libc_r code
    to busy-wait on pipes, causing a major performance hog.

    As an example the patch boosts the multimedia/transcode port (a big
    pipes & threads user) by a factor of 2-3.

    Index: sys_pipe.c
    ===================================================================
    RCS file: /home/ncvs/src/sys/kern/sys_pipe.c,v
    retrieving revision 1.140
    diff -u -r1.140 sys_pipe.c
    --- sys_pipe.c 30 Jul 2003 18:55:04 -0000 1.140
    +++ sys_pipe.c 30 Jul 2003 21:19:41 -0000
    @@ -1041,7 +1041,8 @@
                     if ((space < uio->uio_resid) && (orig_resid <= PIPE_BUF))
                             space = 0;
     
    - if (space > 0 && (wpipe->pipe_buffer.cnt < PIPE_SIZE)) {
    + if (space > 0
    + && wpipe->pipe_buffer.cnt < wpipe->pipe_buffer.size) {
                             if ((error = pipelock(wpipe,1)) == 0) {
                                     int size; /* Transfer size */
                                     int segsize; /* first segment to transfer */ 

    -- 
Pierre Beyssac	      	    pb@fasterix.frmug.org pb@fasterix.freenix.org
    Free domains: http://www.eu.org/ or mail dns-manager@EU.org
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