Re: OT (slightly) swap limits
- From: bob@xxxxxxxxxx (Bob Proulx)
- Date: Sat, 26 Apr 2008 17:13:28 -0600
Damon L. Chesser wrote:
Patrick Ouellette wrote:
Rephrase the question. Ask what the intended use of the machine is,
what response times the users expect, how much real RAM is there, and
what applications/services will run off the machine.
This is a good response. This would be necessary for me to answer the
question. But understanding is still needed to answer well.
Then tell the interviewer how each parameter you've asked about would
influence your decision on how much swap was enough.
As an interviewer I would then propose several different cases and see
how the thought processes went in each of those cases. The person
being interviewed would still be in the hot seat to answer but they
would have gotten good points for knowing that it was a discussion
session and not a strict question-answer session. Personally I am
looking for someone with good general problem solving skills and not
necessarily someone who can solve the particular problems I am posing
to them. No one knows all of the answers. Making use of available
resources is important and don't forget that at that moment your
interviewer is one of your available resources.
How much is enough? As much as the system needs to run and not kill
processes due to lack of memory (real + swap).
I've run machines with 1Gig or more RAM with NO SWAP. I've also run
machines with 4Gig of RAM and 16Gig of swap (BIG datasets).
Each in their own place. But why? What is the formula?
Unfortunately while there are some formula there is also a need for
judgement. There is an old adage, "Good judgement comes from
experience. Experience comes from bad judgement." The old rule was
twice as much swap as ram. Then with Linux on small disks that seemed
harsh and we started to use less swap. How was using less swap on
Linux possible when before on Unix it wasn't? Now the rule is back
again. But why? I know why in my case because it comes from a bad
experience where I set up servers for large memory applications and
Keep reading... I will get to why in a moment. :-)
"At a bare minimum, you need an appropriately-sized root partition, and
a swap partition equal to twice the amount of RAM"
For an enterprise quality server where reliability matters I fully
agree. Yes. You want twice your ram. But you also want to set
vm.overcommit_memory=2 for reliability.
That is RHEL's take on the issue. Still looking for other sources.
Interesting, I think.
Notice that RHEL includes "Enterprise" in their name and they are
catering to the enterprise market.
The problem isn't that the system will slow down to an unusable state
if it actually starts using that much swap. It would. But that isn't
the problem. The problem is that if there isn't enough swap that
Linux will find itself unable to fulfil memory commitments and will
invoke the OOM (out-of-memory) Killer to kill processes until it can
meet commitments. The OOM Killer makes systems unreliable. Avoiding
the OOM killer is needed for highly reliable systems such as in use in
the enterprise environment.
By default Linux will overcommit memory allocation. The malloc(3)
library can't fail. The fork(2) system call can't fail. But this
means that if there isn't enough memory in the future that Linux may
need to kill something. That works similarly to a 'kill -9', the
process simply stops running. There is no ability of the process to
log the event to a log file. It just disappears.
How does the Linux kernel choose which process dies? It guesses based
upon a set of heuristics. Sometimes it guesses right. Sometimes it
guesses wrong. I have personally experienced it killing my X server
while I was using it due to this. That wasn't very pleasant.
Now after learning about this I always disable overcommit. This
restores a traditional Unix process model. Programs that call for
memory (through the malloc() interface or fork(), program level
interfaces that use memory) will see failures if there isn't enough
memory. This is a Good Thing as it allows applications to handle
issues and log them generally deal with the problem.
Back to the original question: Configure virtual memory to be twice
RAM? Or twice the maximum total size of all processes? Really it is
the latter. But RAM sets the capability and so does the amount of
swap so we tune those together to set the maximum capability of the
system even if we are not using it.
I learned about this because I didn't know it when I set up a computer
server pool to be used for some large memory consuming simulations.
They all had 16G of RAM with 64-bit process space. I knew that if
they started swapping that they would run to slowly for us and the
plan was to avoid swapping. If they don't swap they don't need swap
space and so I didn't configure any on them. I came from a Unix
background and didn't expect this different behavior when running on
Linux. We started to see processes randomly stop running. Nothing
was ever logged from the log files for those processes. Large memory
applications never recorded being out of memory. They just stopped
running. A number of folks started cursing Linux for being unreliable
and at that moment for us it was very unreliable! It was from that
experience I learned about the Linux memory overcommit and oom killer
behavior. And with that knowledge I knew that I needed to rebuild the
computer server pool with overcommit off and enough VM.
Summary: So in my mind an Important Aspect of system configuration is
Reliability and in order to get Reliability it is needed to turn off
Linux kernel memory overcommit in order to avoid the dreaded and often
misguided OOM Killer. This is the important point but it affects the
amount of virtual memory required by a system and that in turn affects
the need for having swap space configured. With overcommit turned off
then the rule for twice the amount of RAM is again required. (In
other words you don't need swap / VM configured equal to twice RAM if
vm.overcommit_memory=0 as is the Linux default. But then you must
deal with the OOM killer behavior.)
[ Sidebar: This is how you disable overcommit on a system. This can
be done temporarily with the following command.
sudo sysctl -w vm.overcommit_memory=2
HOWEVER! If you do not have enough virtual memory / swap space at the
time that you activate this configuration then it may cause your
system to be unable to fork() and malloc() starting at that moment.
If the system becomes unusable due to fork failure you may need to
reboot. You must have enough virtual memory configured to handle all
of your expected simultaneously running applications. Be careful that
you can reboot your server remotely if you do not have physical
access. You have been warned!
Since the above 'sysctl' test is a temporary configuration a reboot
will reset to the default allowing memory overcommit and enabling
the out of memory killer if the system runs out of memory and needs
a reboot to be saved. If you decide that is a good configuration to
keep memory overcommit disabled then it may be configured in
/etc/sysctl.conf to be set at boot time. This is what I recommend
and I always configure my servers this way.
For more information on this topic search the web for linux kernel out
of memory killer and memory overcommit and you will find much
discussion about it. Start here:
http://lwn.net/Articles/104179/ (a must read article)
 Full disclosure: This is a previous posting of mine on this topic.
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