cvs commit: src/sys/vm uma_core.c

[Gleb Smirnoff]

  Mohan,

On Wed, Jan 24, 2007 at 10:49:12PM +0000, Mohan Srinivasan wrote:
M> mohans      2007-01-24 22:49:12 UTC
M> 
M>   FreeBSD src repository
M> 
M>   Modified files:
M>     sys/vm               uma_core.c 
M>   Log:
M>   Fix for a bug where only one process (of multiple) blocked on
M>   maxpages on a zone is woken up, with the rest never being woken up as
M>   a result of the ZFLAG_FULL flag being cleared. Wakeup all such blocked
M>   procsses instead. This change introduces a thundering herd, but since
M>   this should be relatively infrequent, optimizing this (by introducing
M>   a count of blocked processes, for example) may be premature.
M>   
M>   Reviewd by: ups@

  I hope this should fix some problems, we are observing. Thanks for
working on this!

  But I wonder, can the following happen. Under high memory pressure,
a zone with a limit is exhausted and several processes are waiting
for memory in this zone. Then a few items are freed, and they are freed
to the per cpu cache, not to the zone cache. Thus, the wakeup() isn't
issued, and waiting processes keep waiting forever. Then, some new
processes can allocate an item from the per cpu cache and free it back
to the per cpu cache, and again waiting processes won't be waked.

I've observed and sshd sleeping on "zonelim", while I was successfully
working via another ssh session.

-- 
Totus tuus, Glebius.
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