Date: Wed, 14 Jul 1999 21:25:31 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: lyndon@orthanc.ab.ca Cc: "Brian F. Feldman" <green@FreeBSD.ORG>, freebsd-hackers@FreeBSD.ORG Subject: Re: Swap overcommit (was Re: Replacement for grep(1) (part 2)) Message-ID: <199907150425.VAA06697@apollo.backplane.com> References: <199907141833.MAA05320@orthanc.ab.ca>
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:Our IMAP server routinely show a footprint of about 1MB private storage.
:This is constant for most operations. However, when you get into doing
:SEARCH and SORT, there are certain cases where we need memory, sometimes
:a *lot* of memory.
:
:Your proposal is that my *well behaved* application should be arbitrarily
:killed, leaving the client stuck with a) no results and b) no IMAP
:connection, in this situation. (And think threaded. That one server
:could be handling *hundreds* of clients.) This is preferable to
:returning a NULL to the malloc() request, which I can handle
:gracefully by simply returning a NO response to the IMAP client?
:
:What it so evil about having a reasonably intelligent malloc() that
:tells the truth, and returns unused memory to the system? Overcommit
:is for lazy programmers, plain and simple. At least the SGI documentation
:about overcommit admits that (or at least, did at one time).
:
:--lyndon
If you are running an IMAP server that regularly runs out of swap
space, you have a configuration problem which needs to be addressed.
It's as simple as that. What you are putting forth is an example
of something that will never happen on a properly configured
server.
In regards to the general case where one is running third-party
applications. Here you are assuming that you can go in and modify
every single piece of software running on the machine to deal
with malloc() returning NULL. Because if you don't, the machine
isn't going to be very stable.
Not only that, you are assuming that you will make the correct
decision on what action to take when malloc() *does* return NULL.
If you decide to return an error code but not exit, what happens
when a potential blowup situation results in thousands of imap
processes being run on the system, and NONE of them exit when
their malloc() fails?
The problem is a whole lot more complex then simply having the
OS return NULL from a malloc(). Currently the OS kills processes
as a last resort. The idea is that no nominally running system
runs out of swap. Now you propose to take away the kernel's
ability to recover some memory as a last resort and instead
put it into the hands of the very user or root-run processes
that are causing the problem in the first place! A much better
solution would be to write a simple watchdog script that notices
when swap space is low and does the right thing -- e.g. kills
the non-essential processes and leaves the essential ones alone.
Then the kernel never actually reaches a state of last-resort.
-Matt
Matthew Dillon
<dillon@backplane.com>
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