RFR: 8244752: Enable Linux support for multiple huge page sizes -XX:LargePageSizeInBytes

[Thomas Stüfe]

Thanks,

sorry for addressing you with the wrong name, I was writing another mail at
the same time and got the names mixed up.

The change looks good now. Thank you for making the comment clearer. I
guess at some point we can try to find a better abstraction for large pages
at the generic os:: namespace level - some platforms have just one, some
have multiple. Of those who have multiple, some have the notion of a
default size, but some (eg AIX, and our closed Hpux port) have not, there,
all page sizes are treated equal.

But this is way out of scope for your proposed change. Thanks for your
patience.

..Thomas


On Thu, May 14, 2020 at 3:44 PM Ivan Walulya <ivan.walulya at oracle.com>
wrote:

> Hi Thomas,
>
> Thanks for the comments
>
>
> 2952 // If MAP_HUGETLB is set, and the system supports multiple huge page
> sizes,
> 2953 // flag bits [26:31] can be used to encode the log2 of the desired
> huge page size.
> 2954 // Otherwise the system's default huge page size will be used.
> 2955 // See mmap(2) man page for more info (since Linux 3.8).
> 2956 // https://lwn.net/Articles/533499/
> <https://urldefense.com/v3/__https://lwn.net/Articles/533499/__;!!GqivPVa7Brio!LmDQYi0jNiIRDJQ6zF29nJULj1sCUcKePWCy8dlGpPKazvDIpkndYUSSAz8-tgpCER8$>
> 2957 #ifndef MAP_HUGE_SHIFT
> 2958   #define MAP_HUGE_SHIFT 26
> 2959 #endif
>
> I may be slow but I needed a while to understand this comment; as a
> standalone comment it is confusing. What flags? Could you clarify the
> comment a bit, especially making clear that "flags" means mmap flags to be
> used in conjunction with MAP_HUGE_TLB?
>
> ---
>
> bool os::Linux::is_valid_large_page_size(size_t large_page_size)
>
>
> Note that os::Linux::is_valid_large_page_size() sounds rather
> inconspicuous; as a caller, without looking at the implementation, I would
> not have assumed it does file IO.
>
>
> Refactored to address the above comments
>
> New webrev: http://cr.openjdk.java.net/~iwalulya/8244752/02/
>
> //Ivan
>