Parallelizing symbol table/string table scan

Karen Kinnear karen.kinnear at
Mon Nov 11 07:46:46 PST 2013

Thank you for asking and for filing RFEs - this is so much better than a webrev
as the first sighting :-)

On Nov 11, 2013, at 8:56 AM, Thomas Schatzl wrote:

> Hi all,
>  recently we (the gc team) noticed severe performance issues with
> symbol table and string table scan during remark.
> Basically, in G1 these pauses are the largest pauses on a reasonably
> tuned system. Also, in particular, symbol table scan alone takes 50% of
> total remark time. String table scan takes another 13%.
> At least symbol table scan is a pretty big issue.
> The simple approach to those is to parallelize these tasks of course,
> however I would like to query you for comments or suggestions :)
> (I am simply throwing some ideas on the wall, in the hope something
> sticks...)
I don't see any reason not to parallelize the scanning.
> One idea that came up to optimize that further has been to not do string
> table or symbol table scrubbing after gc at all if no class unloading
> has been done, assuming that the amount of dead entries are zero anyway.
Just to clarify - there are temp Symbols in the symbol table - so the number
of dead entries with no class unloading will be close to zero, i.e. small enough
that your suggestion of not doing scrubbing unless there has been class loading
makes sense - just don't assume zero.
> This is (imo) true for the string table at least (because they are
> strong roots if not doing class unloading), but I am not so sure about
> the symbol table.
> You probably have more experience about the use of the symbol table, so
> any ideas what could cause symbol table entries to get stale other than
> class unloading, and if so, is this a big concern?
> Another option would be to do this symbol table scrubbing only after a
> certain amount of operations on the symbols, not sure if there is an
> indicator (that does not decrease perf for retrieving too much) for
> that.
> Another idea, again for the symbol table is to scrub it either
> incrementally (eg. depending on available time), or concurrently. I.e.
> some background task periodically waking up and scrubbing (parts of) the
> symbol table.
The tables are read lock-free, with the assumption that they only have
entries removed at a safepoint. If you keep that assumption, you should be
able to do incremental scrubbing.

hope this helps,
> Comments, suggestions?
> I also created a few RFEs for these issues, see
> Symbol table:
> String table:
> Thomas

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