Re: x86: vmalloc and THP

Kirill A. Shutemov
2014-08-12T15:01:55Z

On Tue, Aug 12, 2014 at 05:28:52AM -0700, Eric Dumazet wrote:
> On Tue, 2014-08-12 at 09:07 +0300, Kirill A. Shutemov wrote:
> > On Tue, Aug 12, 2014 at 08:00:54AM +0300, Oren Twaig wrote:
> > >If not, is there any fast way to change this behavior ? Maybe by
> > >changing the granularity/alignment of such allocations to allow such
> > >mapping ?
> >
> > What's the point to use vmalloc() in this case?
>
> Look at various large hashes we have in the system, all using
> vmalloc() :
>
> [ 0.006856] Dentry cache hash table entries: 16777216 (order: 15, 134217728 bytes)
> [ 0.033130] Inode-cache hash table entries: 8388608 (order: 14, 67108864 bytes)
> [ 1.197621] TCP established hash table entries: 524288 (order: 11, 8388608 bytes)

I see lower-order allocation in upstream code. Is it some distribution
tweak?

> I would imagine a performance difference if we were using hugepages.

Okay, it's *probably* a valid point.

The hash tables are only allocated with vmalloc() on NUMA system, if
hashdist=1 (default on NUMA). It does it to distribute memory between
nodes. vmalloc() in NUMA_NO_NODE case will allocate all memory with
0-order page allocations: no physical contiguous memory for hugepage
mappings.

I guess we could teach vmalloc() to interleave between nodes on PMD_SIZE
chunks rather then on PAGE_SIZE if caller asks for big memory allocations.
Although, I'm not sure it it would fit all vmalloc() users.

We also would need to allocate PMD_SIZE-aligned virtual address range
to be able to mapped allocated memory with pmds.

It's *potentially* interesting research project. Any volunteers?