Re: CPU utilisation cap?

From: Sean Chittenden (sean_at_chittenden.org)
Date: 10/21/04

  • Next message: Jason Stone: "Re: decreasing interrupt CPU load" 
    Date: Thu, 21 Oct 2004 10:53:32 -0700
To: lukem.freebsd@cse.unsw.edu.au

    > I am measuring idle time using a CPU soaker process which runs at a
    > very low priority. Top seems to confirm the output it gives.
    >
    > What I see is strange. CPU utilisation always peaks (and stays) at
    > between 80 & 85%. If I increase the amount of work done by the UDP
    > echo program (by inserting additional packet copies), CPU utilisation
    > does not rise, but rather, throughput declines. The 80% figure is
    > common to both the slow and fast PCI cards as well.
    >
    > This is rather confusing, as I cannot tell if the system is IO bound
    > or CPU bound. Certainly I would not have expected the 133/64 PCI bus
    > to be saturated given that peak throughput is around 550Mbit/s with
    > 1024-byte packets. (Such a low figure is not unexpected given there
    > are 2 syscalls per packet).

    There are two things that come to mind. The first being a patch that
    should have been applied in time for 5.2, but I forget the timing of
    the releases.

    http://lists.freebsd.org/mailman/htdig/cvs-src/2003-October/012628.html

    IIRC, there was another commit that made a similar change specifically
    in the handling of UDP packets, such that it used a TAILQ append
    instead of traversing a linked list. For some reason I think this
    happened after 5.2, but I'm not able to find high nor low of the commit
    and could be pulling said memory into existence. Too many commits to
    keep track of.

    Recently rwatson has been doing a bunch of testing of our UDP stack and
    has also committed a fair number of fixes. He likely has many words of
    wisdom on what's changed or not right in 5.2.1. Regardless, I wager
    that if you give RELENG_5_3 a shot, I bet you'd find performance to be
    less surprising. -sc 

    -- 
Sean Chittenden
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