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

From: Mel Gorman
Date: Thu Aug 25 2016 - 06:08:16 EST

On Wed, Aug 24, 2016 at 11:01:43PM -0500, Christoph Lameter wrote:
> On Wed, 24 Aug 2016, Mel Gorman wrote:
> > If/when I get back to the page allocator, the priority would be a bulk
> > API for faster allocs of batches of order-0 pages instead of allocating
> > a large page and splitting.
> >
> OMG. Do we really want to continue this? There are billions of Linux
> devices out there that require a reboot at least once a week. This is now
> standard with certain Android phones. In our company we reboot all
> machines every week because fragmentation degrades performance
> significantly. We need to finally face up to it and deal with the issue
> instead of continuing to produce more half ass-ed solutions.

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.

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.

Batching of tree_lock during reclaim for large files and swapping is also
relatively low hanging fruit that also is doable in a week or two.

A high-order per-cpu cache for SLUB to reduce zone->lock contention is
also relatively low hanging fruit with the caveat it makes per_cpu_pages
larger than a cache line.

If you want to rework the VM to use a larger fundamental unit, track
sub-units where required and deal with the internal fragmentation issues
then by all means go ahead and deal with it.

Mel Gorman