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https://issues.apache.org/jira/browse/HADOOP-3524?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12603850#action_12603850
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Steve Loughran commented on HADOOP-3524:
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on the same machine, try measuring the cost of System.nanoTime(). it may be
lower cost.
many years ago I did some experiments with JNI calling the x86 opcode for the
in-CPU clock; I think it put the round trip time from Java to assembler and
back at about 600 cycles on a PII system. Not great, but not neglible. It may
be cheaper than System.currentTimeMillis(), though it may get confused by CPUs
varying their clock speeds, and virtual machines moving at a very slow clock
rate compared to wall time -you'd have to test that too.
> JobTracker's processHeartbeat() should not call System.currentTimeMillis()
> everytime
> ------------------------------------------------------------------------------------
>
> Key: HADOOP-3524
> URL: https://issues.apache.org/jira/browse/HADOOP-3524
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Reporter: Amar Kamat
> Attachments: CurrentTimeCost.java
>
>
> Consider the following
> {code:title=JobTracker.java|borderStyle=solid}
> private synchronized boolean processHeartbeat(
> TaskTrackerStatus
> trackerStatus, boolean initialContact) {
> String trackerName = trackerStatus.getTrackerName();
> trackerStatus.setLastSeen(System.currentTimeMillis());
> {code}
> Here, the call to {{System.currentTimeMillis()}} on every call to
> {{JobTracker.processHeartbeat()}} might prove costly. While
> testing/benchmarking HADOOP-2119, we recorded that the JobTracker was able to
> serve ~130 tasks/sec. So that means we might make ~130 calls to
> {{System.currentTimeMillis()}} per second. I think in these cases
> (_last-seen-status_ etc) such a high level of accuracy in terms of timestamp
> is unnecessary and hence can be avoided.
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