Debian vs kernel.org kernel -- pty performance
Hi.
Today I posted a question to the kernelnewbies forum
(http://forum.kernelnewbies.org/read.php?9,1122), but perhaps it would
have been smarter to ask my question here.
The standard Debian kernel is fast and my own custom built kernel is
slow for a program I have been developing. Here is a cut'n'paste of
my postings linked above:
Posting #1
! I have been using custom compiled kernels for years, but have hit a
! problem with my kernel that isn't there if I use a standard Debian
! kernel.
!
! 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.
!
! In a rare moment of inspiration, I downloaded Debian's latest kernel
! (my custom one is 2.6.31.5; I think the Debian one was 2.6.26.x, but I
! don't think that's too relevant) and booted with it. Now 100% CPU
! usage, and the program running vastly quicker. Good...
!
! I've compared my .config with Debian's. Vastly different (no great
! surprise), and I've turned a few slightly promising options on or off
! to match. But the resulting kernel still has this delay.
!
! Can anyone suggest where this "hold the buffer for 8ms" behaviour
! might be hidden?
!
! I have stripped down example programs if anyone wants to investigate.
Posting #2
! One more point...
!
! I have just built a (huge) kernel from the kernel.org sources using
! the debian .config. The pty test is still slow. Evidently there is
! some tweak that the Debian people do to the source. I struck something
! similar a few years ago with tmpfs slowing down over time with
! standard sources, but not with RedHat.
!
! I guess I'll download the Debian sources and see what their changelog
! has to say. Anyone got an easier way?
As you can see, I have now thought of an easier way: ask here.
Does a problem with unnecessary delays in scheduling of terminal based
programs ring a bell with anybody?
Happy to provide the sample programs if that is of interest.
Have fun,
Rob.
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