Troubleshooting a ~40-second minor collection

[Vitaly Davidovich]

Just a theory here ... If that copying is bringing data into memory, it
will pollute cpu caches.  When those utilities are context switched out and
in, it's possible they get run on a core different from before, further
polluting caches there (although on a non busy 32 core system,  I don't
know think kernel scheduler should do that).  When gc threads run, it's
possible they'll stall heavily due to cache misses - this will manifest
itself as low user time, low system time, and high real time.

Sent from my phone
On Dec 3, 2013 7:08 AM, "Aaron Daubman" <daubman at gmail.com> wrote:

> Hi Bengt,
>
> Thanks for the pointers - it feels like there may be some hints here as to
> what to look at next!
>
>
>> This GC has has this user/real time info:
>>
>> [Times: user=0.17 sys=0.00, real=3.45 secs]
>>
>> That means that during 3.45 seconds all of the VM threads only got
>> scheduled to actually run on the CPUs for 0.17 seconds. So, it seems like
>> the OS has scheduled the VM threads out for most of that period. The reason
>> for that is most likely that the system is running too many other
>> applications at the same time.
>>
>
>
> When I see these longer pause times its during the high-rate (~100M/s)
> file copy and un-tar onto this system. This uses at most 2 out of the 32
> cores on this server. So I am still very confused as to what would prevent
> the VM threads from being scheduled... anybody have any ideas?  (The server
> runs at a load average of ~2 on a 32 core system)
>
> Would high context switching or some type of I/O wait have any impact on
> long real vs. user GC time here? I'm having a hard time imagining how that
> would happen... especially since all of the memory should be active now...
>
> Thanks again,
>       Aaron
>
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