Linux, hugetlbfs, UseSHM removal?

[Thomas Stüfe]

Thank you, Andrew, for clarifying.

On Mon, Jun 26, 2023 at 1:28 PM Andrew Haley <aph-open at littlepinkcloud.com>
wrote:

> On 6/23/23 13:11, Thomas Stüfe wrote:
> > Hi all,
> >
> > We talked about this [1] and StefanJ opened an issue open for it [2],
> and also I had private talks with various people. I am currently digging
> around in LP initialization again, and getting rid of SHM mode would be so
> nice. Perfect time for it too, just after LTS fork-off.
> >
> > SHM tlbfs mode causes a lot of extra complexity. Since SHM segments can
> only be detached as a whole, they behave more like Windows VirtualSpace
> instead of mmap. As in, the whole mapping cannot be split, cannot be
> partially uncommitted etc. As StefanJ pointed out in [2], it also requires
> more permissions than mmap-tlbfs mode.
> >
> > I have always had the suspicion that this is a dusty corner of hotspot
> that is rarely used and probably broken in many corner cases. For example,
> the sanity checks at VM startup are not correctly implemented, see
> JDK-8310262
>
> Here's the discussion that led to what we have now:
>
> https://mail.openjdk.org/pipermail/hotspot-dev/2011-April/004060.html
>
> Commit 2383:139667d9836a, is credited to
>
> 7034464: Support transparent large pages on Linux
> Summary: Support transparent huge pages on Linux available since 2.6.38
> Reviewed-by: iveresov, ysr
> Contributed-by: aph at redhat.com
>
> The SHM was left in because of older systems, and caution, I think.
>
> Also:
>
>  >>> Yes, but on Linux at least +UseLargePages (using all that SHM stuff)
>  >>> needs special permissions, so it tends not to be used as much as it
>  >>> could.
>

Good point. StefanJ also mentioned this in
https://bugs.openjdk.org/browse/JDK-8261894 .

Cheers, Thomas


> --
> Andrew Haley  (he/him)
> Java Platform Lead Engineer
> Red Hat UK Ltd. <https://www.redhat.com>
> https://keybase.io/andrewhaley
> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/hotspot-dev/attachments/20230626/6bc335ae/attachment.htm>