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https://issues.apache.org/jira/browse/OAK-5469?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15953737#comment-15953737
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Michael Dürig commented on OAK-5469:
------------------------------------
Maybe realted / interesting here: [shared memory and gc
pauses|http://www.evanjones.ca/jvm-mmap-pause.html].
> TarMK: scaling the content
> --------------------------
>
> Key: OAK-5469
> URL: https://issues.apache.org/jira/browse/OAK-5469
> Project: Jackrabbit Oak
> Issue Type: Epic
> Components: segment-tar
> Reporter: Michael Dürig
> Labels: scalability
> Fix For: 1.8
>
> Attachments: segment-per-path.png
>
>
> Production experience has shown that big repositories are prone to thrashing:
> {quote}
> Monitoring showed as massive level of major page faults, load averages
> several times the number of cores, system cpu levels well above 50% and
> extreme levels of IO. As more IOPS was provisioned the instance consumed all
> available IOPS. The TechOps team reported many TB of read IO per hour and
> hardly any write IO.
> Investigation revealed that the repository size was just larger than the
> available RAM on the machine. The instance was running in MMAPED mode and the
> IO was due to major page faults mapping in and out pages of memory. This was
> made worse by transparent huge page settings causing huge pages to be mapped
> proactively on major page faults. Compaction reduced the repository size to
> less than RAM. The TechOps team now monitor the total tar file size and dont
> let it exceed the RAM on the machine, scheduling compactions to keep within
> limits. Since the default to TarMK was to run memory mapped rather than on
> heap, the JVM had no visibility of the mayhem being caused at OS level.
> {quote}
> This epic is all about improving scalability of the TarMK wrt. the content.
> Below are some initial points to consider. Let's create issues and link them
> to this epic as we go.
> * What kind of internal / external monitoring do we need to understand and
> optimally predict thrashing? Can we monitor the working set (active pages)?
> The number of segments in the segment cache might be a good starting point.
> * (How) can we reproduce the thrashing (easily enough)? Can we scale it down
> (i.e. to an instance with littler RAM)?
> * What is the impact of transparent huge pages (and switching it off)? How
> much do we suffer from read amplification? What would be the impact of not
> memory mapping but instead increasing the size of the segment buffer
> accordingly? Both approaches aim at having finer grained control over the
> data actually being loaded into RAM.
> * What other OS level tweaks should / can we look at?
> * Can we reduce the working set by keeping it more compact? E.g. running
> GC/compaction, reducing read amplification (see above), improving
> de-duplication of values, storing values more efficiently (e.g. dates, and
> boolean), can we on the fly compress buffers (e.g. segments)?
> * How do we testing with big repositories?
> * What is a big repository? (Potential target: 100 GB segment store - 500M
> nodes, TBC)
> * What to measure (indicators of size): size on disk (after compaction),
> number of JCR nodes, number of node records (reachable vs. waste)
> * How to measure?
> * {{oak-run debug}} (needs improvements for better scalability)
> * one-line tool to provide all the info?
> * How to obtain big repositories (generate or re-use existing)?
> * What to analyze / monitor / debug?
> * Possible limits: number of nodes (relative to RAM) for which trashing
> starts to occur, max. number of direct children, max. concurrent requests
> during online garbage collection.
> * Platform monitoring:
> * basic: disc size, IO, CPU, memory
> * Asses impact of hardware upgrades on performance. E.g. what impact
> does doubling RAM/IO/CPU have on our test scenarios.
> * in depth: page faults, writes / reads per process, working set of
> nodes, commit statistics, incoming requests vs Oak operations, other hiccups
> * Tools: [Ganglia|http://ganglia.info/],
> [jHiccups|https://github.com/giltene/jHiccup],
> [AppDynamics|https://www.appdynamics.com/]
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