Request for review (S): 7097516 G1: assert(0<= from_card && from_card<HeapRegion::CardsPerRegion) failed: Must be in range.

Bengt Rutisson bengt.rutisson at
Thu Oct 20 08:33:14 UTC 2011

Hi Stefan,

Thanks for you review!

On 2011-10-20 10:21, Stefan Karlsson wrote:
> Hi Bengt,
> On 10/19/2011 11:45 AM, Bengt Rutisson wrote:
>> Hi again,
>> Updated webrev based on comments from Tony:
> Looks good to me.

Here is the final webrev:

Thanks Tony and Stefan. I am all set to push this now.

> But, I find it really odd that Space::is_in_reserved(...) looks at the 
> expanded size when working on Start Continues regions, and 
> HeapRegion::is_in_reserved_raw looks at the "original" region size. To 
> me, it would be more natural if Space::is_in_reserved(...) actually 
> did what one would expect, look at the original heap region size, and 
> then have a somthing like HeapRegion::is_in_reserved_expanded to 
> expand the size for Start Continues regions.

I agree with you. This is actually a conceptual change. We would need to 
make the start humongous regions keep their size and then just add extra 
information to them regarding the end of the last continues humongous 
region. This could maybe be stored as a Space in the start region.

We need to make sure that is_in_reserved() is consistent with other 
methods such as capacity() and containing(). To me it is more natural to 
explicitly state that you are interested in the special case of start 
humongous regions than to have to explicitly state that you are not 
interested in this special case. Also if we would do this change we 
would never have overlapping regions and we would not have to filter out 
continues regions in places where we have to do that today.

I discussed this with Tony a couple of days ago, so I know he disagrees. 
;-) I'll let him state his argument.

Anyway, such a change should be handled in a CR of its own. I'll go 
ahead and push my small change now.


> StefanK
>> Bengt
>> On 2011-10-19 09:14, Bengt Rutisson wrote:
>>> Hi all,
>>> Could I have a couple of reviews of this change?
>>> Some background:
>>> The humongous start regions in G1 gets their _end marker set to the 
>>> end of the last humongous continues region for the humongous object. 
>>> This means that the start region overlaps all of the continues 
>>> regions. Code that handles regions need to be aware of this special 
>>> case.
>>> However, PerRegionTable::add_reference_work() had a subtle issue 
>>> with this special case. Here is what happened:
>>> A thread, TA, looks up the PerRegionTable, PRT,  for a particular 
>>> heap region. This heap region happens to be a humongous continues 
>>> region. Let's call it HRA.
>>> Another thread, TB, wants to look up the PerRegionTable for the 
>>> humongous start region of that same humongous object. Let's call 
>>> that heap region HRB. If we are unlucky TB is the first thread that 
>>> wants the PerRegionTable for HRB and thus has to allocate it. Now, 
>>> there is a limit to how many PerRegionTables we allow in the system. 
>>> If this limit has been reached TB will find a suitable existing 
>>> PerRegionTable and coarsen that to use a bitmap for the remset 
>>> instead of the PerRegionTable.
>>> This means that TB can actually "steal" the PRT from underneath TA.
>>> TA is aware that this can happen. So, before it tries to do any work 
>>> with the information in the PRT it tries to verify that it hasn't 
>>> been "stolen" (actually coarsened). It does this by storing the heap 
>>> region for the PRT in a local variable (called loc_hr) and then 
>>> check that the reference that it is about to handle still belong to 
>>> the heap region.
>>> Basically:
>>>     HeapRegion* loc_hr = hr();
>>>     if (loc_hr->is_in_reserved(from)) {
>>>        ...
>>>     }
>>> Normally this is safe. If TA gets HRB in loc_hr the test will fail. 
>>> However, in the case where HRB is the humongous start region the 
>>> test will not fail. Instead we will pass the test and execute the 
>>> code inside the if. There we try to calculate a card index based on 
>>> the bottom of the heap region and the from address. Now this card 
>>> index will be larger than the number of cards we have per region, 
>>> since from actually is not in the region.
>>> The fix is to introduce a new method called 
>>> HeapRegion::is_in_reserved_raw() and use this in the test. The _raw 
>>> method will not return false even for humongous start objects in the 
>>> example above.
>>> I picked the name is_in_reserved_raw() to be consistent with the 
>>> heap_region_containing()/heap_region_containing_raw() methods. I am 
>>> not particularly fond of the name, so if there are better 
>>> suggestions I am happy to change it.
>>> Testing
>>> I was able to reproduce the issue on bur398-216 after 40-50 
>>> iterations. With my fix it has now ran 540 iteration without failing.
>>> Other uses of is_in_reserved()
>>> Tony and I went through the G1 code and looked for other uses of 
>>> HeapRegion::is_in_reserved(). As far as we can tell the other uses 
>>> all avoid the issue that PerRegionTable::add_reference_work() has. 
>>> Most of them because they are used during evacuation where humongous 
>>> objects are not present at all.
>>> Webrev:
>>> CR:
>>> 7097516 G1: assert(0<= from_card && 
>>> from_card<HeapRegion::CardsPerRegion) failed: Must be in range.
>>> Thanks,
>>> Bengt

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