Re: [patch] SLQB slab allocator

[Christoph Lameter <cl@linux-foundation.org>]

On Thu, 15 Jan 2009, Nick Piggin wrote:

> Definitely it is not uncontrollable. And not unchangeable. It is
> about the least sensitive part of the allocator because in a serious
> workload, the queues will continually be bounded by watermarks rather
> than timer reaping.

The application that is interrupted has no control over when SLQB runs its
expiration. The longer the queues the longer the holdoff. Look at the
changelogs for various queue expiration things in the kernel. I fixed up a
couple of those over the years for latency reasons.

> > Object dispersal
> > in the kernel address space.
>
> You mean due to lower order allocations?
> 1. I have not seen any results showing this gives a practical performance
>    increase, let alone one that offsets the downsides of using higher
>    order allocations.

Well yes with enterprise app you are likely not going to see it. Run HPC
and other low latency tests (Infiniband based and such).

> 2. Increased internal fragmentation may also have the opposite effect and
>    result in worse packing.

Memory allocations in latency critical appls are generally done in
contexts where high latencies are tolerable (f.e. at startup).

> 3. There is no reason why SLQB can't use higher order allocations if this
>    is a significant win.

It still will have to move objects between queues? Or does it adapt the
slub method of "queue" per page?

> > Memory policy handling in the slab
> > allocator.
>
> I see no reason why this should be a problem. The SLUB merge just asserted
> it would be a problem. But actually SLAB seems to handle it just fine, and
> SLUB also doesn't always obey memory policies, so I consider that to be a
> worse problem, at least until it is justified by performance numbers that
> show otherwise.

Well I wrote the code in SLAB that does this. And AFAICT this was a very
bad hack that I had to put in after all the original developers of the
NUMA slab stuff vanished and things began to segfault.

SLUB obeys memory policies. It just uses the page allocator for this by
doing an allocation *without* specifying the node that memory has to come
from. SLAB manages memory strictly per node. So it always has to ask for
memory from a particular node. Hence the need to implement memory policies
in the allocator.

> > Even seems to include periodic moving of objects between
> > queues.
>
> The queues expire slowly. Same as SLAB's arrays. You are describing the
> implementation, and not the problems it has.

Periodic movement again introduces processing spikes and pollution of the
cpu caches.

> There needs to be some fallback cases added to slowpaths to handle
> these things, but I don't see why it would take much work.

The need for that fallback comes from the SLAB methodology used....

> > SLQB maybe a good cleanup for SLAB. Its good that it is based on the
> > cleaned up code in SLUB but the fundamental design is SLAB (or rather the
> > Solaris allocator from which we got the design for all the queuing stuff
> > in the first place). It preserves many of the drawbacks of that code.
>
> It is _like_ slab. It avoids the major drawbacks of large footprint of
> array caches, and O(N^2) memory consumption behaviour, and corner cases
> where scalability is poor. The queueing behaviour of SLAB IMO is not
> a drawback and it is a big reaon why SLAB is so good.

Queuing and the explosions of the number of queues with the alien caches
resulted in the potential of portions of memory vanishing into these
queues. Queueing means unused objects are in those queues stemming from
pages that would otherwise (if the the free object would be "moved" back
to the page) be available for other kernel uses.

> > If SLQB would replace SLAB then there would be a lot of shared code
> > (debugging for example). Having a generic slab allocator framework may
> > then be possible within which a variety of algorithms may be implemented.
>
> The goal is to replace SLAB and SLUB. Anything less would be a failure
> on behalf of SLQB. Shared code is not a bad thing, but the major problem
> is the actual core behaviour of the allocator because it affects almost
> everywhere in the kernel and splitting userbase is not a good thing.

I still dont see the problem that SLQB is addressing (aside from code
cleanup of SLAB). Seems that you feel that the queueing behavior of SLAB
is okay.

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