FAQ

On 27.2.2012 12:57, Tomas Vondra wrote:
On 27 ?nor 2012, 11:26, Peter Kjellstr?m wrote:
On Sunday 26 February 2012 19.59.07 Tomas Vondra wrote:
...
i.e. about 200 MB of free memory, but apache fails because of segfaults
when forking a child process:

[16:49:51 2012] [error] (12)Cannot allocate memory: fork: Unable to
fork new process
[16:51:17 2012] [notice] child pid 2577 exit signal Segmentation
fault (11)
In general things can get quite bad with relatively high memory pressure
and
no swap.
Sure, but there's no such pressure. There was almost 200MB of "free"
memory (used for page cache, not dirty thus easy to drop).
That said, one thing that comes to mind is stacksize. When forking the
linux
kernel needs whatever the current stacksize is to be available as (free +
free
swap).

Also, just because you see Y bytes free doesn't mean you can successfully
malloc that much (fragmentation, memory zones, etc.).
Yup, I'm aware of that. But it's rather improbable, especially given the
other symptoms.

Update: After submitting the original post, I've noticed that these issues
probably started about a week ago after upgrading a kernel and several
related packages. I've had a swap there and the issues were not as severe,
so I haven't noticed that before. I do remember I got an OOM error during
that upgrade and I thought I've dealt with it properly, but maybe not. So
I've reinstalled (remove+install) all those packages, rebooted and the
problems disappeared. I will check that in the evening, but hopefully it's
fixed.
Well, I've found the actual issue. It clearly was my stupidity as I was
messing with overcommit_memory without fully understanding it.

What I did was that I set (as mentioned in the original post)

vm.overcommit_memory = 2

which limits the amount of available memory to

swap + vm.overcommit_ratio * RAM

where vm.overcommit_ratioP by default, so you can allocate swap + 1/2
the physical memory. This is just fine if you have a swap - for example
if you have swap size equal to RAM, this means 150% of RAM is available
for processes.

The issues start when you disable swap (as I did) - then it effectively
limits the available memory to 50% of physical RAM (and receive OOM if
you try to allocate more. This is exactly what happened to me :-(

So what I did was that I set

vm.overcommit_ratio = 100

which gives me 100% of RAM. I know this will give me an OOM if I use all
the physical RAM, but that's expected - I don't want to use swap on a
virtual machine with poor I/O (and the services are set accordingly).

So the moral is don't mess with something you don't fully understand.

kind regards
Tomas

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postedFeb 26, '12 at 1:59p
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