Re: [bisected] pty performance problem

[Ingo Molnar <mingo@elte.hu>]

(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.
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