[tor-relays] Out of memory message

Nick Mathewson nickm at freehaven.net
Mon Dec 15 18:43:11 UTC 2014


On Wed, Dec 10, 2014 at 2:31 AM, Roger Dingledine <arma at mit.edu> wrote:
> On Sun, Dec 07, 2014 at 01:43:46PM +0100, Logforme wrote:
>> To me it looks like an attacker that ramped up over a 6 hour period and
>> then stopped building new circuits. Since the tor process still uses all
>> available memory (more than 24 hours later) I guess the attacker still
>> holds some circuits open.
>
> Careful with your conclusion there -- because of memory fragmentation, the
> process can still hold the memory even when Tor has freed the memory. That
> happens because some part of the memory page is in use and some is freed,
> but since not all of it is freed the allocator doesn't take it back.
>
> Some quick searches point to
> https://www.ibm.com/developerworks/community/blogs/kevgrig/entry/linux_native_memory_fragmentation_and_process_size_growth
> as what looks like a nice summary of the issue.
>
> --Roger

If this *is* memory fragmentation, one possible reason why it would
have started appearing recently might be that we turned the freelist
code off by default in 0.2.5, on the theory that it should be safer to
do that than not.

https://trac.torproject.org/projects/tor/ticket/11476 has more
background here.  I don't think it's the likeliest explanation, but
it's worth examining.

Are the people experiencing this issue using similar allocators?

-- 
Nick


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