RFR: 8273872: ZGC: Explicitly use 2M large pages

[Thomas Schatzl]

On Thu, 16 Sep 2021 07:59:32 GMT, Per Liden <pliden at openjdk.org> wrote:

> ZGC requires large pages to be 2M. However, ZGC doesn't explicitly asks for this page size and instead relies on the default large pages size for the system to be 2M. On systems where this is not true, ZGC will fails with an error message. To avoid this, ZGC should explicitly ask for 2M large pages and not rely on the system default. Furthermore, ZGC currently ignores `-XX:LargePageSizeInBytes`. ZGC should fails with an error message if it's specified to something other than 2M.

Looks good apart from the minor typos.

src/hotspot/os/linux/gc/z/zPhysicalMemoryBacking_linux.cpp line 210:

> 208: 
> 209:   // Create file
> 210:   const int extra_flags = ZLargePages::is_explicit() ? (MFD_HUGETLB | MFD_HUGE_2MB) : 0;

Potentially the use of the constant `MFD_HUGE_2MB` could be generalized a little and calculated from a required page size like we do in `Linux::commit_memory_special` via a helper function like `os::Linux::hugetlbfs_page_size_flag`; at least that extra flag just looks like it is actually generated the same way. Looking through `memfd.h` it is *exactly* the same as for the corresponding `HUGETLB_*` flags.
But since this is really ZGC specific code, up to you.

src/hotspot/os/linux/gc/z/zPhysicalMemoryBacking_linux.cpp line 216:

> 214:     log_debug_p(gc, init)("Failed to create memfd file (%s)",
> 215:                           (ZLargePages::is_explicit() && (err == EINVAL || err == ENODEV)) ?
> 216:                           "Hugepages (2M) not supported" : err.to_string());

Maybe this should be something like:

Suggestion:

                          "Hugepages (2M) not available" : err.to_string());


As in `ZArguments` the code already checks that 2M page size are requested (if any).

src/hotspot/share/gc/z/zArguments.cpp line 75:

> 73:   }
> 74: 
> 75:   // Only 2M large pages is supported

Suggestion:

  // Only 2M large pages are supported.

src/hotspot/share/gc/z/zArguments.cpp line 77:

> 75:   // Only 2M large pages is supported
> 76:   if (!FLAG_IS_DEFAULT(LargePageSizeInBytes) && LargePageSizeInBytes != 2 * M) {
> 77:     vm_exit_during_initialization("Invalid -XX:LargePageSizeInBytes (only 2M large pages is supported)");

Suggestion:

    vm_exit_during_initialization("Invalid -XX:LargePageSizeInBytes (only 2M large pages are supported)");

-------------

Marked as reviewed by tschatzl (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5541