RFR (M/L): 8010722 assert: failed: heap size is too big for compressed oops

Thomas Schatzl thomas.schatzl at oracle.com
Wed Aug 28 06:32:46 PDT 2013


Hi all,

On Tue, 2013-05-14 at 16:37 +0200, Thomas Schatzl wrote:
> Hi all,
> 
JIRA:
> https://jbs.oracle.com/bugs/browse/JDK-8010722

 can I have reviews for the following change?

> 
> In argument processing related to ergonomically determining compressed
> oops use, there were a few use-before-set issues leading to crashes that
> this fix tries to resolve.
> 
> bugs.sun.com
> http://bugs.sun.com/view_bug.do?bug_id=8010722
> 
> JIRA:
> https://jbs.oracle.com/bugs/browse/JDK-8010722
> 
> webrev:
> http://cr.openjdk.java.net/~tschatzl/8010722/webrev.1/

There is a new webrev at
http://cr.openjdk.java.net/~tschatzl/8010722/webrev.2 ; this has become
necessary after the changes in CR 8007074 (Stefan K's large pages
changes) and CR 8003424 from Harold.

Particularly the latter changed how the class metadata space is
allocated, removing a lot of additional checking and alignment issues.

However the core issue still persists, the maximum heap size for
compressed oops is calculated wrongly as the NULL page is not properly
accounted for.

testing:
jprt, jprt test cases

Thomas

> Here's a walkthrough of the changes:
> 
> As mentioned, the cause of this CR is that ergonomics for determining
> the maximum java heap size usable for compressed oops uses variables
> that are later changed ergonomically.
> 
> It is best to look at the changes beginning from
> Arguments::set_ergonomics_flags(): the idea of this change is to avoid
> later overflow, so the change tries to conservatively estimate sizes of
> the non-java heap parts. The complication is that not even the later
> effective alignment of these heap parts has been determined at this
> point.
> 
> So the change first calculates the maximum possible heap alignment by
> calling set_max_heap_alignment(); this size is influenced by OS page
> sizes, so the change needs to initialize large pages by calling
> os::large_page_init() in Arguments::parse(), before the call to
> set_ergonomics_flags(). The maximum possible alignment is then
> calculated by asking the active GC for its maximum alignment, as at this
> point the GC has already been determined, the maximum page size, and
> other requirements, like alignment for card table size etc.
> 
> Now the code can calculate the conservative estimate for actual maximum
> heap for compressed oops used in set_use_compressed_oops(), by
> subtracting the conservatively aligned sizes of the other heap parts.
> (In Arguments::max_heap_for_compressed_oops()) The result is the maximum
> possible heap that can use compressed oops, minus the aligned metaspace
> size, minus the aligned null page size.
> 
> There is another circular dependency problem here, the metaspace size itself is later ergonomically sized; the change fixes this problem by anticipating that in Arguments::max_heap_for_compressed_oops(), using the same predicate for determining whether to ergonomically size it or not [this is CR8009778 I think].
> 
> The other changes are straightforward: the os_* changes result from that
> large page initialization must be done earlier now; the changes in the
> collectors themselves are simply about providing the collector's maximum
> alignment. The change in Universe::reserve_heap() contains one assertion that checks whether the conservative estimate for alignment has been conservative enough earlier.
> 
> The test case tests test cases from the CR that work now, and additional
> border cases related to ergonomically deciding heap size for compressed
> oops.
> 
> One side effect of this change is that the ergonomically determined heap size is slighly smaller now (so that it always fits :).
> 
> Thanks,
>   Thomas
> 
> 
> 
> 
> 




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