RFR(L): 8201572: Improve Metaspace Statistics

Thomas Stüfe thomas.stuefe at gmail.com
Mon Apr 23 16:32:13 UTC 2018

Hi all,

may I please have opinions and reviews for the following improvement.

Bug: https://bugs.openjdk.java.net/browse/JDK-8201572
Webrev: http://cr.openjdk.java.net/~stuefe/webrevs/8201572-improve-metaspace-reporting/webrev.00/webrev/

(Note: it needs patch for 8202074 [3] to apply cleanly)

This is a combined feature-add and cleanup for the metaspace
reporting. It should have no effect on metaspace allocation itself.

This change does a number of things:

- It improves the "VM.metaspace" jcmd, making it hopefully much more versatile.

- It improves the way statistics are collected: The number of times we
walk the CLDG or the internal metaspace structures is reduced. It also
adds another global counter to MetaspaceUtils.

- Though this was not primarily intended as a cleanup change, a number
of cleanups are part of this change. A number of functions were
removed. Where new code was added, I attempted to define and follow a
new scheme using namespaces and separate files. See below for details.



-> Improvements of jcmd:

The jcmd "VM.metaspace" command now features a number of sub command
which control the details printed:

jcmd VM.metaspace [basic] [show-loaders] [by-chunktype] [by-spacetype]
[vslist] [vsmap] [scale]

All of them except for "basic" can be combined with each other.

  shows a basic report; it needs no CLDG walk and no locks, so it
should be safe under most conditions.
  this shows metaspace usage by loaders (basically what VM.metaspace
did before - this was originally added by Zhengyu Gu)
  Numbers are broken down by chunk type (small, medium...) in addition
to total numbers
  Numbers are broken down by space type (bootloader, anonymous,
reflection...) in addition to total numbers
  The virtual space list is printed in detail
  Prints the virtual space map (ascii map of chunk placements)

When no arguments are given, a summary is displayed.

In addition to the above mentioned features we now print
- a "Waste" section detailing wastage.
- in debug mode, a section with some internal counters. This was just
curiosity on my part, I wanted to see how "hot" some of the metaspace
allocation paths actually get.
- We now count overhead too, i.e. space needed for chunk headers (was
counted as "used" before)
- We now also show deallocated space (block freelist)

Some output examples:

<No arguments>





"show-loaders" and "by-chunktype combined"




-> The way we collect statistics was streamlined. I tried to get rid
of all functions which were walking structures to just sum up a single
value; not only was that unnecessary code duplication, it was also

This means most of the SpaceManager::sum_xxxx() functions are gone, as
well as all the different MetaspaceUtils::xxx_xxx_slow() functions.

They were replaced by functions named "collect_statistics" or
"add_statistics", which accumulate all needed data in one go into a
statistics structure. This is more efficient since we usually need
more than one value and so it makes sense to collect them in one go.

The newly added statistics objects are:

- UsedChunksStatistics
- SpaceManagerStatistics
- ClassLoaderMetaspaceStatistics

They live in a new file (memory/metaspace/metaspaceStatistics).

I also ripped out the preexisting ChunkManagerStatistics, modified it
to follow the same convention, and moved it into this new file.

-> We now have a new central reporting function,
MetaspaceUtils::print_report() - this was once Zhengyu's
print_for_nmt() function, but I expanded it and renamed it to be the
central entry point. Reporting options are handed into it via flags.

In addition to that, we have a new
MetaspaceUtils::print_basic_report(), which prints a short summary
under the promise of not walking or locking anything. This is used at
error reporting time to print a short summary to the hs-err file. It
is also used for VM.info().

-> All new functions now accept the "scale" parameter known from NMT.
I expanded it a bit to have a "dynamic" setting. In that setting, the
printer chooses the best unit for a given value. I find it works quite
well, so I made it the default. I also changed printing to correctly
print very-small-but-not-0 numbers or percentages, where before they
were rounded to 0. See linked output examples.

-> Other things to note:

- MetaspaceUtils::dump was used to print some details at Metaspace
OOM. This now calls MetaspaceUtils::print_basic_report() directly, and
I removed the dump() function.

- I moved printing the summary out of the PrintCLDMetaspaceInfoClosure
and into the print_report function. PrintCLDMetaspaceInfoClosure still
prints the individual loader metaspace usage numbers.

- I streamlined naming of values somewhat. Where we refer to in-use
chunks, I now use consistently "capacity", "free", "waste", "used" and
the new "overhead", and the assumption is that capacity is the sum of
all the other values.

- About the newly added namespaces: Since the patch is already quite
big, I did not want to load it with more cleanup. However, neither did
I want to just dump the new classes and functions into metaspace.cpp.
So as a compromise I tried to follow a new scheme with newly added
code. If you agree on this organization, this may be the way to split
metaspace.cpp in some follow up change.

The idea was to move all metaspace-internal stuff into
memory/metaspace and hide it from the outside. Also, I did put
metaspace coding into an own namespace ("metaspace") and
metaspace-internal coding into "metaspace::internal". This is just a
proposal, we can bike shed. Future changes could move more classes out
of metaspace.cpp into metaspace/ and the metaspace::internal namespace
to make metaspace.cpp malleable again.

Thank you,

Kind Regards, Thomas

[1] https://bugs.openjdk.java.net/browse/JDK-8201572
[2] http://cr.openjdk.java.net/~stuefe/webrevs/8201572-improve-metaspace-reporting/webrev.00/webrev/
[3] http://cr.openjdk.java.net/~stuefe/webrevs/8202074-metaspace-retire-after-humongous-chunks-are-allocated/webrev.00/webrev/

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