Re: phk malloc, was (Re: ptmalloc2)
:Let's stop here then. The feature I am looking for doesn't come with
:Unix by default, beaten horse, etc.
Any machine which is capable of paging data out to disk is going to
slow down once the load exceeds a certain point... that is, once the
working set exceeds the amount of main memory available. This is not
the point where memory failures occur, this is the point where the
machine starts to page to swap. As the load increases past this point
the paging load to swap increases dramatically. SWAP does not have to
fill up for the machine to become highly inefficient from load. What
is going to happen is that the efficiency of the services running on the
machine will go down. An email proxy, for example, which is capable of
handling 200 connections per second may wind up being able to handle
only 50 connections per second.
This has nothing at all to do with having an overcommit knob. An
overcommit knob DOES NOT SAVE YOU from thrashing the machine. Modern
machines configured with sufficient swap space will die a slow death
long before swap fills up... long, long before an overcommit knob would
come into play. Overcommit is not a measure of the working set, it's a
measure of avaiable SWAP space + available main memory. It WILL NOT make
programs run more efficiently, and it certainly will not make things
more reliable.
I hope I'm getting through to you here. You do understand that your
email proxy program is going to run the machine into the ground from
paging long before turning off any overcommit knob would actually
have an effect, don't you?
Sysops always tune their machines to keep the working set within
reasonable bounds, so the machine stays efficient. It is fairly
obvious when a machine becomes inefficient due to paging load. Memory
allocation and overcommit has nothing at all to do with the problem.
-Matt
Matthew Dillon
<dillon@backplane.com>
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