RFR: 8261023: Document why memory pretouch must be a store [v2]

[Aleksey Shipilev]

On Wed, 3 Feb 2021 11:28:57 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   may I have reviews for this additional comment that explains why `os::pretouch_memory` needs to use a store and must not use a read which would be more convenient?
>> 
>> Basically on some (all?) OSes memory pages are only actually backed with physical memory on a store to that page. Before that a common "zero page" may be used to satisfy reads. This is not what is intended here.
>> 
>> A previous comment (that has been removed long ago) seems to have been a bit confused about the actual issue:
>> 
>> -      // Note the use of a write here; originally we tried just a read, but
>> -      // since the value read was unused, the optimizer removed the read.
>> -      // If we ever have a concurrent touchahead thread, we'll want to use
>> -      // a read, to avoid the potential of overwriting data (if a mutator
>> -      // thread beats the touchahead thread to a page).  There are various
>> -      // ways of making sure this read is not optimized away: for example,
>> -      // generating the code for a read procedure at runtime.
>> 
>> It indicates that the reason for using a store has been that the compiler would optimize away the reads (which begs the question why a `volatile` read has not been used).
>> 
>> Maybe these zero page optimizations came later than that original implementation though.
>> 
>> Testing: local compilation - it's adding a comment only, really.
>> 
>> Thanks,
>>   Thomas
>
> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
> 
>   shade review

Marked as reviewed by shade (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/2373