[lttng-dev] High memory consumption issue on RCU side

[Evgeniy Ivanov]

Hi Mathieu,

On Sat, Sep 24, 2016 at 12:59 AM, Mathieu Desnoyers
<mathieu.desnoyers at efficios.com> wrote:
> ----- On Sep 22, 2016, at 3:14 PM, Evgeniy Ivanov lolkaantimat at gmail.com wrote:
>
>> Hi all,
>>
>> I'm investigating high memory usage of my program: RSS varies between
>> executions in range 20-50 GB, though it should be determenistic. I've
>> found that all the memory is allocated in this stack:
>>
>> Allocated 17673781248 bytes in 556 allocations
>>        cds_lfht_alloc_bucket_table3     from liburcu-cds.so.2.0.0
>>        _do_cds_lfht_resize      from liburcu-cds.so.2.0.0
>>        do_resize_cb             from liburcu-cds.so.2.0.0
>>        call_rcu_thread          from liburcu-qsbr.so.2.0.0
>>        start_thread             from libpthread-2.12.so
>>        clone                    from libc-2.12.so
>>
>> According pstack it should be quiescent state.  Call thread waits on syscall:
>> syscall
>> call_rcu_thread
>> start_thread
>> clone
>>
>> We use urcu-0.8.7, only rculfhash (QSBR). Is it some kind of leak in
>> RCU or any chance I misuse it? What would you recommend to
>> troubleshoot the situation?
>
> urcu-qsbr is the fastest flavor of urcu, but it is rather tricky to use well.
> Make sure that:
>
> - Each registered thread periodically reach a quiescent state, by:
>   - Invoking rcu_quiescent_state periodically, and
>   - Making sure to surround any blocking for relatively large amount of
>     time by rcu_thread_offline()/rcu_thread_online().
>
> In urcu-qsbr, the "default" state of threads is to be within a RCU read-side.
> Therefore, if you omit any of the two advice above, you end up in a situation
> where grace periods never complete, and therefore no call_rcu() callbacks can
> be processed. This effectively acts like a big memory leak.

It was the original assumption, but in memory stacks I don't see such
allocations for my data. Instead huge allocations happen right in
call_rcu_thread. Memory footprint for my app is about 20 GB, erasing
RCU data is a rare operation, so almost 20 GB in rcu thread looks
suspecios. I'll try to not erase any RCU protected data and reproduce
the issue (complicated thing is that under memory tracer it happens
not so often).


> Hoping this helps,
>
> Thanks,
>
> Mathieu
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com



-- 
Cheers,
Evgeniy