Re: Should calculation of vm.overcommit_ratio be changed?

Re: Should calculation of vm.overcommit_ratio be changed?

[Dirk Geschke]

Hi all,

I am not on the mailing list and a friend pointed me to this
thread...

Probably we had the same problem: We had a linux computer with
16 GB of RAM without swap. There was only one big job running
on it which did a lot of I/O. This program failed to allocate
much memory, we thought that this was due to the high amount
of cached memory use. To avoid problems with overcommit we had
set overcommit_memory to 2.

Now I have seen this thread and now it gets clear: The default
value of overcommit_ratio is 50, therefore one program can not
allocate more than 8GB of memory at all.

After reading this thread I wrote a little programm to allocate
memory in 512MB blocks and fill it with zeros. My test system
has 4GB of RAM and so I started:

qfix:~# free
             total       used       free     shared    buffers     cached
Mem:       4052376     338124    3714252          0          0      17992
-/+ buffers/cache:     320132    3732244
Swap:            0          0          0

geschke@qfix:~$ ./a.out
got 1 * 512MB
got 2 * 512MB
got 3 * 512MB
malloc failure after 3 * 512 MB

So 1.5 GB are ok, 2 GB of possible 4GB not. I guess some memory of
the 4GB are not useable at all and therefore the limit is slightly
below 2GB with an overcommit_ratio=50.

Next step is to set overcommit_ratio=100:

qfix:~# echo 100 >/proc/sys/vm/overcommit_ratio

and run the porgram again:

geschke@qfix:~$ ./a.out
got 1 * 512MB
got 2 * 512MB
got 3 * 512MB
got 4 * 512MB
got 5 * 512MB
got 6 * 512MB
malloc failure after 6 * 512 MB

That are more than 3 GB but I would have expected to get at least
3.5GB:

geschke@qfix:~$ free
             total       used       free     shared    buffers     cached
Mem:       4052376     344976    3707400          0          0      22472
-/+ buffers/cache:     322504    3729872
Swap:            0          0          0

Maybe this is due to a reserved percentage for the root user?

However, if I set overcommit_ratio=110 I get more than 3.5 GB:

geschke@qfix:~$ ./a.out
got 1 * 512MB
got 2 * 512MB
got 3 * 512MB
got 4 * 512MB
got 5 * 512MB
got 6 * 512MB
got 7 * 512MB
malloc failure after 7 * 512 MB

However, now I tested this with a high usage of cached memory, I
die a lot of I/O:

geschke@qfix:~$ free
             total       used       free     shared    buffers     cached
Mem:       4052376    1945512    2106864          0          0    1621200
-/+ buffers/cache:     324312    3728064
Swap:            0          0          0

A new run gives:

geschke@qfix:~$ ./a.out
got 1 * 512MB
got 2 * 512MB
got 3 * 512MB
got 4 * 512MB
got 5 * 512MB
got 6 * 512MB
got 7 * 512MB
malloc failure after 7 * 512 MB

and:

qfix:~# free
             total       used       free     shared    buffers     cached
Mem:       4052376     346928    3705448          0          0      26008
-/+ buffers/cache:     320920    3731456
Swap:            0          0          0

So the cached memory is not really a problem for malloc.

But since I am testing, I tried what will happens if a lot of memory
is already in use. So I opened a large file with "vi":

geschke@qfix:~$ free
             total       used       free     shared    buffers     cached
Mem:       4052376    1597168    2455208          0          0     391364
-/+ buffers/cache:    1205804    2846572
Swap:            0          0          0

Now I start the program again:

geschke@qfix:~$ ./a.out
got 1 * 512MB
got 2 * 512MB
got 3 * 512MB
got 4 * 512MB
got 5 * 512MB
malloc failure after 5 * 512 MB

Fine: It seems that there is not really a problem to increase to
overcommit_ratio=100 if there is no swap in the system and one
has set overcommit_memory=2.

So I think, it is not really a problem to run with these settings.

Best regards

Dirk
-- 
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| Dr. Dirk Geschke       / Plankensteinweg 61    / 85435 Erding        |
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| dirk@geschke-online.de / dirk@lug-erding.de  / kontakt@lug-erding.de | 
+----------------------------------------------------------------------+

Re: Should calculation of vm.overcommit_ratio be changed?

[Dave Wright]

Thanks for your reply Alan.

>
> Sounds like the distribution should be tuning the value according to the [available memory?]

Yes, that's one approach and I'll probably recommend that to some
folks, however that would probably just be set at install time, and
there's no guarantee that memory/swap amounts wouldn't change later
(e.g. they added more RAM, and now find they can't "use" it because
overcommit ratio is too low).

>> max commit = min(swap, ram) * overcommit_ratio + max(swap, ram) ?
>>
>
> Which is wromg - some of your RAM ends up eaten by the kernel, by
> pagetables and buffers etc. 50% is probably very conservative but the
> point of VM overcommit is exactly that - and you end up deploying swap
> as a precaution against disaster rather than because you need it.
>

I actually think the current formula does the reverse - rather than
seeing swap as an overrun area, it includes the full amount of swap in
the max commit then adds only a percentage of main memory. I'm not
sure what the original motivation for that was - perhaps preventing a
page-file backed mmap from exhausting physical memory as well?

Setting overcommit to 100 in the absence of swap probably isn't a good
idea, however the default of 50 when there is less swap than RAM is a
problem.

I'm sure there will be resistance to any suggestion about changing the
calculation, since it works fine as long as you know about it and set
it properly for your situation, but I do think a more sensible default
can be found.
My first suggestion was above. Other possible options include:
1. Just changing the default % from 50 to 90

2. max commit = (ram + swap) * overcommit_ratio
[with a default ratio of 90% or more]

3. max commit = ram + swap + overcommit_bytes
[overcommit_bytes is a fixed number of bytes, rather than a
percentage, and can be negative to increase safety or positive to
allow aggressive overcommit]

Any of these options would increase the VM space (and thus usable RAM)
for scenarios where you had more RAM than swap. For scenarios where
you had more swap than RAM, they would allow more of it to be
committed than currently, but you're well into swap at that point
already so it's unlikely that it would hurt performance at all. Any of
them could still be manually tweaked to get a specific result, but the
starting value would make sense in a wider range of conditions.


-Dave Wright

Re: Should calculation of vm.overcommit_ratio be changed?

[Alan Cox]

> might have 4GB RAM and 1GB swap. I don't think you would expect
> Desktop users to understand or tweak overcommit_ratio, but I also
> don't think having the distro simply change the default from 50 (to
> 100 or something else) would cover all the cases well.

Sounds like the distribution should be tuning the value according to the
> 
> Would it make more sense to have the overcommit formula be calculated as:
> 
> max commit = min(swap, ram) * overcommit_ratio + max(swap, ram) ?
> 
> When swap>=ram, the formula works exactly the same as it does now, but
> when ram>>swap, you are guaranteed to always be able to your full RAM
> (even when swap=0).

Which is wromg - some of your RAM ends up eaten by the kernel, by
pagetables and buffers etc. 50% is probably very conservative but the
point of VM overcommit is exactly that - and you end up deploying swap
as a precaution against disaster rather than because you need it.

Should calculation of vm.overcommit_ratio be changed?

[Dave Wright]

The current calculation of VM overcommit (particularly with default
vm.overcommit_ratio==50) seems to be a hold-over from the days when we
had more swap than physical memory. For example, 1/2 phy mem + swap
made sense when you had a 1GB of memory and 2GB of swap, however I
recently ran into an issue on a server that had 8GB RAM and 2GB swap.
The OOM killer was getting triggered as VM commit hit 6GB, even though
there was plenty of RAM available. Once I figured out what was going
on, I manually tweaked the ratio to be 110%.
It looks like current distro recommendations are still "have as much
swap as you have RAM", in which case the current calculation is fine,
but with SSD becoming more common on boot drives, I think many users
will end up with less swap than RAM - consider a desktop user who
might have 4GB RAM and 1GB swap. I don't think you would expect
Desktop users to understand or tweak overcommit_ratio, but I also
don't think having the distro simply change the default from 50 (to
100 or something else) would cover all the cases well.

Would it make more sense to have the overcommit formula be calculated as:

max commit = min(swap, ram) * overcommit_ratio + max(swap, ram) ?

When swap>=ram, the formula works exactly the same as it does now, but
when ram>>swap, you are guaranteed to always be able to your full RAM
(even when swap=0).

-Dave Wright