Re: what is the purpose of SLAB and SLUB (was: Re: [PATCH v3] mm/slab: Improve performance of gathering slabinfo) stats

[Mel Gorman <mgorman@suse.de>]

On Thu, Aug 25, 2016 at 02:55:43PM -0500, Christoph Lameter wrote:
> On Thu, 25 Aug 2016, Mel Gorman wrote:
> 
> > Flipping the lid aside, there will always be a need for fast management
> > of 4K pages. The primary use case is networking that sometimes uses
> > high-order pages to avoid allocator overhead and amortise DMA setup.
> > Userspace-mapped pages will always be 4K although fault-around may benefit
> > from bulk allocating the pages. That is relatively low hanging fruit that
> > would take a few weeks given a free schedule.
> 
> Userspace mapped pages can be hugepages as well as giant pages and that
> has been there for a long time. Intermediate sizes would be useful too in
> order to avoid having to keep lists of 4k pages around and continually
> scan them.
> 

Userspace pages cannot always be mapped as huge or giant. mprotect on a
4K boundary is an obvious example.

> > Dirty tracking of pages on a 4K boundary will always be required to avoid IO
> > multiplier effects that cannot be side-stepped by increasing the fundamental
> > unit of allocation.
> 
> Huge pages cannot be dirtied?

I didn't say that, I said they are required to avoid IO multiplier
effects. If a file is mapped as 2M or 1G then even a 1 byte write requires
2M or 1G of IO to writeback.

> This is an issue of hardware support. On
> x867 you only have one size. I am pretty such that even intel would
> support other sizes if needed. The case has been repeatedly made that 64k
> pages f.e. would be useful to have on x86.
> 

64K pages are not a universal win even on the arches that do support them.

-- 
Mel Gorman
SUSE Labs

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