GuaranteedSafepointInterval clarification

[Vitaly Davidovich]

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.

sent from my phone
On May 28, 2015 9:25 PM, "David Holmes" <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
>>
>>