Long young generation GC?

Y. S. Ramakrishna y.s.ramakrishna at oracle.com
Tue Oct 26 16:34:40 UTC 2010

On 10/26/10 07:07, Hi Guava wrote:
> Here is additional information about the machine running the JVM. It
> is a virtual machine running in a private cloud. Could it be something
> like swapping that caused problem?

Not swapping, but perhaps the management of "time" perhaps in a virtualized
setting (by that i mean that there may be interactions between the
host/hypervisor and the guest OS that could cause the JVM to observe
time jumps of this sort)? I'd suggest gathering more data on its
reproducibility (or otherwise) in both a VM and non-VM setting.

Over to the time experts in the runtime team who may have encountered
issues in VM settings previously. (I have heard of occasional such reports in
virtual settings before but don't know if any of these were definitively chased
down.) You might also want to check with the VM provider to see if they
might know of such issues.

-- ramki

> On Mon, Oct 25, 2010 at 8:51 PM, Y. Srinivas Ramakrishna
> <y.s.ramakrishna at oracle.com> wrote:
>> On 10/25/2010 5:49 PM, Y. Srinivas Ramakrishna wrote:
>>> On 10/25/2010 5:32 PM, Hi Guava wrote:
>>>> The third young generation GC took 439.2720750 secs but the user and
>>>> real time are only 0.08 seconds. What does it mean?
>>> The machine may be using NTP, and the time may have been changed?
>> Seems a rather large jump, so may not be NTP (which i am told uses
>> adjtime() to slowly accelerate the time forward or decelerate it backward),
>> but rather an abrupt perhaps manual change in TOD.
>> Over to the experts....
>>> JVM timestamps on Linux seem still to be based on TOD rather than
>>> on TSC. Someone in the runtime team (cc'd) may have more detail on
>>> why that might still be so.
>>> -- ramki
>>>> 72667.213: [GC 72667.213: [ParNew: 38336K->4224K(38336K), 1.2473840 secs]
>>>> 3443948K->3420569K(6549376K), 1.2474290 secs] [Times: user=0.64 sys=1.10,
>>>> real=1.25 secs]
>>>> 72680.531: [GC 72680.532: [ParNew: 38336K->4221K(38336K), 0.2916570 secs]
>>>> 3008948K->2979033K(6549376K), 0.2916710 secs] [Times: user=0.26 sys=0.03,
>>>> real=0.29 secs]
>>>> 72681.425: [GC 72681.426: [ParNew: 38325K->4224K(38336K), 439.2720750
>>>> secs] 3013053K->2979055K(6549376K), 439.2720750 secs] [Times: user=0.08
>>>> sys=0.00, real=0.08 secs]
>>>> Environment:
>>>> Java HotSpot(TM) 64-Bit Server VM Version 1.6.0_20-b02
>>>> Linux Version 2.6.18-128.1.1.el5 on amd64
>>>> -Xms6400m
>>>> -Xmx6400m
>>>> -Xss256k
>>>> -XX:+UseConcMarkSweepGC
>>>> -XX:+PrintGCDetails
>>>> -XX:+PrintGCTimeStamps
>>>> -XX:+UseCompressedOops
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