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From "Daniel Templeton (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8873) throttle directoryScanner
Date Wed, 23 Sep 2015 20:48:04 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8873?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14905222#comment-14905222

Daniel Templeton commented on HDFS-8873:

bq. Unlike the modulo solution, it does not have a pathological case where we never get to
run at all despite a non-zero throttle rate.

I don't see the pathological case.  Assume the throttle is 1ms and we always oversleep:

.0000 - thread calls throttle(), no block
-- thread runs --
.0422 - thread calls throttle(), sleep for 588ms
.1999 - thread wakes up from oversleep, run limit this second set to 1000ms
-- thread runs (999 < 1000) --
.2190 - thread calls throttle(), sleep for 810ms
.3106 - thread wakes up from oversleep, run limit this second set to 107ms
-- thread runs (106 < 107) --

The throttle() method is guaranteed to exit when at least (1000 - limit) ms have passed and
the calling thread is scheduled again.  What am I missing?

As fas as I can tell, the difference between the StopWatch approach and the modulo approach
is the case when a thread wakes up within (1000 - limit) of the end of the second.  In that
case, the modulo approach will allow the thread to run longer than the StopWatch approach
would.  In other words, the modulo approach is focused on trying to hold to the per-second
duty cycle, and the StopWatch approach is trying to hold to the duty cycle regardless of second
boundaries.  Given that neither the sleep time nor run time is reliable, I don't see where
it matters much one way or the other.

That said, I did just notice that this patch is broken in the case of spurious wake-ups. 
I'll have a new patch for that (including tests) shortly.

bq. I'm confused about this code...

Good catch.  Copy-paste error, grabbed the wrong key.

> throttle directoryScanner
> -------------------------
>                 Key: HDFS-8873
>                 URL: https://issues.apache.org/jira/browse/HDFS-8873
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: datanode
>    Affects Versions: 2.7.1
>            Reporter: Nathan Roberts
>            Assignee: Daniel Templeton
>         Attachments: HDFS-8873.001.patch, HDFS-8873.002.patch, HDFS-8873.003.patch, HDFS-8873.004.patch,
HDFS-8873.005.patch, HDFS-8873.006.patch
> The new 2-level directory layout can make directory scans expensive in terms of disk
seeks (see HDFS-8791) for details. 
> It would be good if the directoryScanner() had a configurable duty cycle that would reduce
its impact on disk performance (much like the approach in HDFS-8617). 
> Without such a throttle, disks can go 100% busy for many minutes at a time (assuming
the common case of all inodes in cache but no directory blocks cached, 64K seeks are required
for full directory listing which translates to 655 seconds) 

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