RFR: 8272807: Permit use of memory concurrent with pretouch

[Kim Barrett]

On Fri, 4 Feb 2022 11:07:52 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> [Trying again, after a couple previous proposals.]
>> 
>> Please review this change to os::pretouch_memory to permit use of the memory
>> being pretouched concurrently with the pretouch operation.  This will allow
>> future changes that can make use of such overlap.
>> 
>> Testing:
>> mach5 tier1-3 with -XX:+AlwaysPreTouch.
>> 
>> Verified no performance impact by comparing startup times with Epsilon GC and
>> -XX:+AlwaysPretouch on a large enough heap that the pretouch takes a
>> significant amount of time.  Tested on both linux-x64 and linux-aarch64.
>> 
>> None of these test concurrent use; that will wait for future changes that use
>> the new feature.
>
> src/hotspot/share/runtime/os.cpp line 1771:
> 
>> 1769:       Atomic::add(reinterpret_cast<int*>(cur), 0, memory_order_relaxed);
>> 1770:       if (cur == last) break;
>> 1771:     }
> 
> Took me some time to check the loop. Suggestion:
> 
> 
>     do {
>       Atomic::add(reinterpret_cast<int*>(cur), 0, memory_order_relaxed);
>       cur += page_size;
>     } while (cur < last);

Intentionally didn't write it that way, because of the potential for UB pointer arithmetic overflow in `cur += page_size`.

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PR: https://git.openjdk.java.net/jdk/pull/7343