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

Tony Printezis tony.printezis at
Fri Oct 21 10:47:39 UTC 2011

On 10/20/2011 04:31 PM, Stefan Karlsson wrote:
> I understand how reusing capacity(), used(), etc. could have made 
> parts of the G1 code easier to write. Is it worth it, though?

...compared to what? Always checking whether a region is humongous or 
not? Absolutely. This is something that's worked out nicely in G1 so 
far. The only aspect of this I'd change would be for the region 
iterators to skip continues humongous regions, given that we filter them 
out most of the time anyway.

> Could you point me to some of the usages in G1 where we rely on 
> capacity(), used(), etc. to report the "extended" range of Start 
> Humonguos regions?

Look at when we add a region to one of the heap region sets. We do check 
whether it's humongous or not but that's to calculate the number of 
regions it spans. In fact, maybe we'd be better off just calculating 
that as capacity() / RegionSize and skip the test. In fact, this is a 
nice example of how this approach simplifies the code.

An additional point: when we set up the block offset table for a 
humongous region, all the entries on it have to point to the bottom of 
the starts humongous region (as the rest do not have an object start), 
so it's again natural for that whole area to be covered by that region 
wrt to used and capacity.


> thanks,
> StefanK
>> Note, however, that the only time we really need to "original" 
>> version of the above methods is in the current RSet code given that 
>> each RSet "chunk" corresponds to exactly one region. Everything else 
>> in the code relies on the fact that the above methods on SH regions 
>> are the "extended" versions. So, we save ourselves a lot of trouble 
>> but not having to check whether a region is SH or not before querying 
>> it.
>> I'd also like to point out that I'd like us to re-architect the RSets 
>> so that they do not rely on regions but, instead, on (fixed size) 
>> memory ranges (could be equal to the region size, could be smaller, 
>> could be larger). That way, we'll eliminate the need for the "raw" 
>> version of is_in_reserved() and deal with SH and non-humongous 
>> regions uniformly.
>> Tony
>> On 10/20/2011 04:21 AM, 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.
>>> 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.
>>> 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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