Cost of single-threaded nmethod hotness updates at each safepoint (in JDK 8)

Vitaly Davidovich vitalyd at gmail.com
Fri Jul 31 22:10:51 UTC 2015


Yes, I misremembered thinking turning off tiered disabled sweeps at
safepoints; sorry for the noise.

sent from my phone
On Jul 31, 2015 6:04 PM, "Srinivas Ramakrishna" <ysr1729 at gmail.com> wrote:

>
> Hi Bernd --
>
> It doesn't seem coupled to GC; here's an example snapshot:
>
> sun.ci.codeCacheCapacity=13041664
> sun.ci.codeCacheMaxCapacity=50331648
> sun.ci.codeCacheMethodsReclaimedNum=4281
> sun.ci.codeCacheSweepsTotalNum=409
> sun.ci.codeCacheSweepsTotalTimeMillis=1541
> sun.ci.codeCacheUsed=10864704
>
> sun.gc.collector.0.invocations=6319
> sun.gc.collector.1.invocations=6
>
> BTW, to a question of Vitaly's on this thread earlier, this code executes
> irrespective of whether you are running tiered or not (the above is from
> tiered explictly off  -- earlier ones were with tiered on --, yet note the
> excessive time in the stack walk as part of this code in JDK 8):
>
> [mark nmethods, 0.0171828 secs]
>
> On a similar note (and to Vladimir's earlier note of turning off code
> cache flushing) and with the understanding that there were lots of changes
> related to tiered compilation and code cache flushing between 7 ad 8,
> turning on Tiered in 7 leads to the occasional (rare) long safepoint for
> the stack walks, but else not.
>
> And finally the same issue must exist in 9 as well, albeit based on code
> inspection, not running with JDK 9 yet.
> Have a good weekend!
> -- ramki
>
> On Fri, Jul 31, 2015 at 2:44 PM, Bernd Eckenfels <ecki at zusammenkunft.net>
> wrote:
>
>> Am Fri, 31 Jul 2015 12:07:18 -0700
>> schrieb Srinivas Ramakrishna <ysr1729 at gmail.com>:
>>
>> > sun.ci.codeCacheSweepsTotalNum=58
>> ...
>> > Notice that the code cache usage is less that 35 MB, for the 240 MB
>> > capacity, yet it seems we have had 58 sweeps already
>>
>> I would also be interested in what causes this. Is this caused by
>> System.gc maybe? (we do see sweeps and decreasing code cache usage on
>> systems where no pressure should exist).
>>
>> Gruss
>> Bernd
>>
>
>
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