G1 question: concurrent cleaning of dirty cards

John Cuthbertson john.cuthbertson at oracle.com
Fri Jun 28 23:06:43 UTC 2013

Hi Igor.

You misunderstood me. I meant  that if we use safepoints to refine cards 
all of the code that currently supports refinement by mutators can be 
removed. That' all.


On 6/28/2013 4:02 PM, Igor Veresov wrote:
> The mutator processing doesn't solve it. The card clearing event is 
> still asynchronous with respect to possible mutations in other 
> threads. While one mutator thread is processing buffers and clearing 
> cards the other can sneak in and do the store to the same object that 
> will go unnoticed. So I'm afraid it's either a store-load barrier, or 
> we need to stop all mutator threads to prevent this race, or worse..
> igor
> On Jun 28, 2013, at 1:53 PM, John Cuthbertson 
> <john.cuthbertson at oracle.com <mailto:john.cuthbertson at oracle.com>> wrote:
>> Hi Igor,
>> Yeah G1 has that facility right now. In fact you added it. :) When 
>> the number of completed buffers is below the green zone upper limit, 
>> none of the refinement threads are refining buffers. That is the 
>> green zone upper limit is number of buffers that we expect to be able 
>> to process during the GC without it going over some percentage of the 
>> pause time (I think the default is 10%). When the number of buffers 
>> grows above the green zone upper limit, the refinement threads start 
>> processing the buffers in stepped manner.
>> So during the safepoint we would process N - green-zone-upper-limit 
>> completed buffers. In fact we could have a watcher task that monitors 
>> the number of completed buffers and triggers a safepoint when the 
>> number of completed buffers becomes sufficiently high - say above the 
>> yellow-zone upper limit.
>> That does away with the whole notion of concurrent refinement but 
>> will remove a lot of the nasty complicated code that gets executed by 
>> the mutators or refinement threads.
>> My main concern is that the we would be potentially increasing the 
>> number and duration of non-GC safepoints which cause issues with 
>> latency sensitive apps. For those workloads that only care about 90% 
>> of the transactions this approach would probably be fine.
>> We would need to evaluate the performance of each approach.
>> The card cache delays the processing of cards that have been dirtied 
>> multiple times - so it does act kind of like a buffer reducing the 
>> potential for this issue.
>> JohnC
>> On 6/28/2013 12:47 PM, Igor Veresov wrote:
>>> The impact on the next collection however can be bounded. Say, if 
>>> you make it have a safepoint to reap the buffers when the number of 
>>> buffer reaches $n$, that alone would put a cap on the potential 
>>> pause incurred during the collection. The card cache currently has 
>>> the same effect, sort of, right?
>>> igor
>>> On Jun 28, 2013, at 12:26 PM, John Cuthbertson 
>>> <john.cuthbertson at oracle.com <mailto:john.cuthbertson at oracle.com>> 
>>> wrote:
>>>> Hi Igor,
>>>> On 6/28/2013 9:47 AM, Igor Veresov wrote:
>>>>> On Jun 28, 2013, at 7:08 AM, "Doerr, Martin" <martin.doerr at sap.com 
>>>>> <mailto:martin.doerr at sap.com>> wrote:
>>>>>> Hi Igor,
>>>>>> we didn’t find an easy and feasible way to ensure the ordering, 
>>>>>> either.
>>>>>> Grabbing the buffers and cleaning the cards at safepoints might 
>>>>>> be the best solution.
>>>>> Would anybody from the G1 team like to think about that?
>>>> I've been thinking about this issue on an off for the last few 
>>>> weeks when I get the time. I mentioned it to Vladimir a couple of 
>>>> times to get his input.
>>>>>> Maybe removing the barrier that flushes the store to the 
>>>>>> cardtable makes the problem more likely to occur.
>>>>>> I guess the purpose of the barrier was exactly to avoid this problem
>>>>>> (which should be working perfectly if the post barriers had 
>>>>>> StoreLoad barriers, too).
>>>>> Yeah, but like you noted that would have a horrific effect on 
>>>>> performance. So, it's probably best to bunch the work up to at 
>>>>> least eliminate the need of extra work when, say, you're looping 
>>>>> and storing to a limited working set (G1 uses the cardtable 
>>>>> basically for that purpose). The safepoint approach will likely 
>>>>> require more memory for buffers and the load will be spiky, and if 
>>>>> the collection were to happen right after we grabbed the buffers 
>>>>> the collector will have to process all of them which is not going 
>>>>> to work well for predictability. But nothing better comes to mind 
>>>>> at this point.
>>>>> Btw, there are already periodic safepoints to do bias locking 
>>>>> revocations, so may be it would make sense to piggyback on that.
>>>> Piggy backing on all the other safepoint operations might work if 
>>>> they happen frequently enough but I don't know if that 's the case. 
>>>> And as you, even then there will be times where we haven't had a 
>>>> safepoint for a while and will have a ton of buffers to process at 
>>>> the start of the pause.
>>>> It might be worth adding a suitable memory barrier to the G1 post 
>>>> write barrier and evaluating the throughput hit.
>>>> JohnC

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