RFR: 8272807: Permit use of memory concurrent with pretouch

[Kim Barrett]

On Fri, 4 Feb 2022 12:18:27 GMT, Thomas Stuefe <stuefe 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 1759:
> 
>> 1757:   assert(is_power_of_2(page_size), "page size misaligned: %zu", page_size);
>> 1758:   assert(page_size >= sizeof(int), "page size too small: %zu", page_size);
>> 1759:   if (start < end) {
> 
> Should we assert start <= end, or maybe even start < end?

I waffled over asserting start <= end, so have no objection to making that change.  In testing I ran across cases of start == end, though haven't pursued them to see if they might indicate a problem or just a reasonable boundary case.

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