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Subject[bisected] pty performance problem
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.

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 <>
! 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 <>
! Signed-off-by: Linus Torvalds <>

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,


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