Re: Problems with huge_pages and IBM Power8

On 2016-04-12 21:58:14 +0200, reiner peterke wrote:
> Hi
>
> We have been doing some testing with Postgres (9.5.2) compiled on a Power8 running Centos 7
>
> When working with huge_pages, we initially got this error.
>
> munmap(0x3efbe4000000) failed: Invalid argument

*munmap*, not mmap failed? that's odd; because there the hugepagesize
shouldn't have much of an influence. If something fails it should be the
initial mmap. Could you show a strace of a failed start with an
unmodified postgres?

> after a bit of investigation we noticed that hugepagesize is har coded
> to 2MB

Note it's not actually hardcoded to some size. It's just about rounding
the size to a multiple of 2MB due to an older kernel bug:
/*
* Round up the request size to a suitable large value.
*
* Some Linux kernel versions are known to have a bug, which causes
* mmap() with MAP_HUGETLB to fail if the request size is not a
* multiple of any supported huge page size. To work around that, we
* round up the request size to nearest 2MB. 2MB is the most common
* huge page page size on affected systems.

> Going further, we tried testing hugepages also on Ubuntu 16.04, also on the power8. On Ubuntu Postgres did not like the hugepages at all (set also to 16MB) and consistently crashed.

> Looking for some insight into this issue. the error from the postgres
> log on ubuntu is below. It apperas to be related to semephores.

I've a bit of a hard time believing that this is related to huge pages.

Greetings,

Andres Freund