GuaranteedSafepointInterval clarification

[David Holmes]

On 29/05/2015 11:52 PM, Vitaly Davidovich wrote:
> Hi David,
>
> Thanks for the response.  What resource shortage are you referring to
> wrt GSI? I'd actually like to increase the interval as having the VM
> thread wake up every 1s and get CPU time (imagine a case where the JVM
> has been affinitized to a subset of CPUs on the machine) is undesirable
> as it may interfere with critical app threads.  Hence seemed like a
> legit tunable given the very low default.

It is mainly the code-cache sweeping, but then as a side-effect you get 
the monitor cleanup and some other things I can't recall of the top of 
my head.

You can change the interval you just need to add the 
UnlockDiagnosticVMOptions flag to do so.

David
-----

> sent from my phone
>
> On May 28, 2015 9:25 PM, "David Holmes" <david.holmes at oracle.com
> <mailto:david.holmes at oracle.com>> wrote:
>
>     Hi Vitaly,
>
>     I'll try to answer your questions but these are just my
>     views/understanding of things.
>
>     On 29/05/2015 6:45 AM, Vitaly Davidovich wrote:
>
>         Hi all,
>
>         Could someone explain the reasoning for 1s being the default
>         value here? It
>         seems quite low given the types of things it appears to handle
>         (monitor
>         deflation, inline cache updates, symbol table rehashing, nmethod
>         sweeping,
>         etc).
>
>
>     A balance between not inducing STW safepoints too frequently, while
>     still ensuring timely cleanup of specific resources in cases where
>     it is not happening by virtue of other safepoint related VM
>     operations occurring.
>
>         Also, I do not always see these safepoints initiated every ~1s.
>         Looking at
>         SafepointSynchronize::is_cleanup_needed(), this seems to kick in
>         only when
>         InlineCacheBuffer has some entries.  Is that right? This code
>         will only run
>         if compiler has updated inline caches?
>
>
>     Although there are a number of cleanup actions that will occur at a
>     safepoint, only the InlineCacheBuffer::is_empty() related cleanup is
>     deemed necessary to run at least once a second. The other cleanup
>     actions are assumed to be less critical, and normally handled by the
>     "natural" occurrence of safepoint VM operations in the execution of
>     an application.
>
>         Why is GuaranteedSafepointInterval a diagnostic option and not
>         full product
>         like, say, a GC tuning option?
>
>
>     GC tuning knobs are expected to turned by end users in the normal
>     course of business. Tweaking the GSI is not something that many
>     people ever have to adjust, and it would generally be done as part
>     of diagnosing a resource shortage of some kind.
>
>     Cheers,
>     David
>
>         Finally, when tracing safepoint stats, these cleanup ones appear
>         as "no vm
>         operation" in the output -- it seems like it would be nicer to print
>         something like "GuaranteedSafepointInterval" or something to
>         that effect.
>
>         Thanks
>