for review (M): 6863023: need non-perm oops in code cache for JSR 292
Thomas.Rodriguez at Sun.COM
Tue Aug 25 09:24:42 PDT 2009
On Aug 25, 2009, at 1:42 AM, John Rose wrote:
> On Aug 11, 2009, at 11:31 AM, Tom Rodriguez wrote:
>> I don't really like the non_perm naming being baked into this. I
>> expect at some point we'll want/need to fix this to have a more
>> refined notion of whether the GC needs to scan the nmethods that
>> won't be based on is_perm.
> OK, that's fair.
>> I expect that a lot of nmethods will start ending up on the list
>> and scanning a significant portion of the code cache twice for
>> every scavenge seems like an overhead we'd want to avoid.
> The new root list is used instead of walking the whole code cache.
> Both scans don't need to happen in the same operation. So I don't
> get what you mean here by "scanning twice".
Once for marking and once to update any pointers.
>> I don't think your marking changes are right. I think the new rule
>> has to be that during a scavenge a the non-perm subset of nmethods
>> are scanned as strong roots and during a full collection all
>> nmethods are treated as weak roots as they are right now. We could
>> make them be weak during a scavenge but then we'd need to perform
>> the do_unloading/make_unloaded logic during a scavenge and it seems
>> like we should avoid that. Given this I think the call to
>> non_perm_nmethod_oops_do in psMarkSweep is wrong since it makes
>> them into strong roots. As far as I can tell the others are
>> probably right though I don't claim to understand the layering in
>> our gcs.
> That's a good catch. Thanks for explaining it. I agree that making
> those nmethods strong roots will inhibit class unloading for the
> affected code (that which uses invokedynamic).
> Most of the extra oops we're putting into the nmethods are also
> rooted in the method's constant pool, so they won't go dead until
> the enclosing nmethod also needs to get unloaded. Some of the extra
> oops might be rooted uniquely in the nmethod, because they are
> optimistic tests on cases that no longer exist. This is similar to
> what happens to our type-predicted virtual calls, when the predicted
> type disappears. At that point, the nmethod holds the last
> remaining reference to the predicted type, and needs to be flushed.
> There is a lighter-weight case where an inline cache has a stale
> pointer; the IC gets reset, instead of the whole nmethod getting
> unloaded. In the case of a predicted invokedynamic target, the
> whole nmethod needs to be flushed.
> Does this sound reasonable? If so, I just need to remove the call
> in phase 1 of psMarkSweep to non_perm_nmethod_oops_do, and let
> nmethods with stale oops get flushed.
Yes that sounds right.
> Thanks for the review. I've updated the webrev:
> -- John
>> On Jul 22, 2009, at 1:53 AM, John Rose wrote:
>>> In order to inline method handles at invokedynamic instructions,
>>> it is necessary to manipulate MethodHandle and CallSite constants
>>> from generated code. Since these objects are created by ordinary
>>> user code and subject to usual GC, they are not preallocated in
>>> the perm gen.
>>> Currently compiled code requires that any oop embedded as an
>>> instruction constant or any other nmethod part must be
>>> OopDesc::is_perm. For example, internal method objects and
>>> classes and interned strings are permanent so that they can be
>>> manipulated from compiled code.
>>> This restriction is a bug for JSR 292. Luckily, there is a simple
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