Messages in this thread | | | Date | Sun, 22 Nov 2009 07:39:26 +0100 | From | Ingo Molnar <> | Subject | Re: [bisected] pty performance problem |
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(Cc:-ed Alan and Linus - mail repeated below. eot.)
* Robert Swan <swan.r.l@gmail.com> wrote:
> I posted this to the kernel-newbies list, but have graduated to the > adults forum: > > ! Two C programs are having a query-response conversation through a > ! pseudo terminal: > ! > ! A (client) -- forever { send query; read response } > ! B (server) -- forever { read query; send response } > ! > ! Neither has any I/O apart from the pty conversation, so I'd expect to > ! see CPU usage at 100%. When I ran it, the CPU was pretty well idle. > ! After a fair bit of fiddling, it turned out that both sides were > ! taking about 8ms for their read() calls. At that point it seemed > ! pretty clear that this was a delay in the kernel, not the code. > ! > [snip] > > 2.6.31-rc2-00205-gb4b21ca good > 2.6.31-rc2-00206-gd945cb9 bad > > and still bad with the latest: 2.6.32-rc8-00011-ga8a8a66 > > the git log says: > ! commit d945cb9cce20ac7143c2de8d88b187f62db99bdc > ! Author: Alan Cox <alan@linux.intel.com> > ! Date: Tue Jul 7 16:39:41 2009 +0100 > ! > ! pty: Rework the pty layer to use the normal buffering logic > ! > ! This fixes the ppp problems and various other issues with call locking > ! caused by one side of a pty called in one locking context trying to match > ! another with differing rules on the other side. We also get a big slack > ! space to work with that means we can bury the flow control deadlock case > ! for any conceivable real world situation. > ! > ! Signed-off-by: Alan Cox <alan@linux.intel.com> > ! Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> > > I can provide reasonably stripped down code which demonstrates the > problem. It has been reproduced by one other person, though his delay > was about 2ms. > > Have fun, > > Rob. > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.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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