Re: [patch] SLQB slab allocator

[Nick Piggin]

On Thu, Jan 15, 2009 at 02:05:55PM -0600, Christoph Lameter wrote:
> On Thu, 15 Jan 2009, Nick Piggin wrote:
> 
> > > The higher orders can fail and will then result in the allocator doing
> > > order 0 allocs. It is not a failure condition.
> >
> > But they increase pressure on the resource and reduce availability to
> > other higher order allocations. They accelerate the breakdown of the
> > anti-frag heuristics, and they make slab internal fragmentation worse.
> > They also simply cost more to allocate and free and reclaim.
> 
> The costs are less since there is no need to have metadata for each of
> the 4k sized pages. Instead there is one contiguous chunk that can be
> tracked as a whole.

In terms of memory footprint of metadata, SLQB and SLUB both use the
struct page for this, so no advantage. In terms of performance, well
it seems that SLUB is not faster with higher order allocations, but
slower with order-0 allocations. So this seems to just be an argument
for using higher order allocations in SLUB, rather than an argument
against using order-0 in SLQB or SLAB>

 
> > > Higher orders are an
> > > advantage because they localize variables of the same type and therefore
> > > reduce TLB pressure.
> >
> > They are also a disadvantage. The disadvantages are very real. The
> > advantage is a bit theoretical (how much really is it going to help
> > going from 4K to 32K, if you still have hundreds or thousands of
> > slabs anyway?). Also, there is no reason why the other allocators
> > cannot use higher orer allocations, but their big advantage is that
> > they don't need to.
> 
> The benefit of going from 4k to 32k is that 8 times as many objects may be
> handled by the same 2M TLB covering a 32k page. If the 4k pages are
> dispersed then you may need 8 2M tlbs (which covers already a quarter of
> the available 2M TLBs on nehalem f.e.) for which the larger alloc just
> needs a single one.

Yes I know that. But it's pretty theoretical IMO (and I could equally
describe a theoretical situation where increased fragmentation in higher
order slabs will result in worse TLB coverage).

Has there been a demonstrated advantage of this? That outweighs the
costs of using higher order allocs? If so, then I could just add some
parameters to SLQB to allow higher order allocs as well.

 
> > I'd like to see any real numbers showing this is a problem. Queue
> > trimming in SLQB can easily be scaled or tweaked to change latency
> > characteristics. The fact is that it isn't a very critical or highly
> > tuned operation. It happens _very_ infrequently in the large scheme
> > of things, and could easily be changed if there is a problem.
> 
> Queue trimming can be configured in the same way in SLAB. But this means
> that you are forever tuning these things as loads vary. Thats one of the

As I said, this is not my experience. Queue trimming is a third order
problem and barely makes any difference in a running workload. It just
serves to clean up unused memory when slabs stop being used. There is
basically no performance implication to it (watermarks are far more
important to queue management of a busy slab).

Tens of millions of objects can be allocated and freed between queue
trimming intervals.


> frustrations that led to the SLUB design. Also if the objects in queues
> are not bound to particular page (as in slub) then traversal of the queues
> can be very TLB fault intensive.

Yeah but it happens so infrequently it should be in the noise. I have
never seen any real problems caused by this. Have you?

 
> > What you have in SLUB IMO is not obviously better because it effectively
> > has sizeable queues in higher order partial and free pages and the
> > active page, which simply never get trimmed, AFAIKS. This can be harmful
> > for slab internal fragmentation as well in some situations.
> 
> It has lists of free objects that are bound to a particular page. That
> simplifies numa handling since all the objects in a "queue" (or page) have
> the same NUMA characteristics.

The same can be said of SLQB and SLAB as well.

> There is no moving between queues
> (there is one exception but in general that is true) because the page list can
> become the percpu list by just using the pointer to the head object.

Exactly the same way SLQB can move a queue of remotely freed objects back
to the allocating processor. It just manipulates a head pointer. It doesn't
walk every object in the list.


> Slab internal fragmentation is already a problem in SLAB. The solution
> would be a targeted reclaim mechanism. Something like what I proposed in
> with the slab defrag patches.
 
It is still a much bigger problem with 16x larger pages. If you have targetted
reclaim and 32K slabs, then you will still have a bigger problem than
targetted reclaim and 4K slabs (and the targetted reclaim will be less
efficient because it will have to free more objects).


> There is no need for trimming since there is no queue in the SLAB sense. A
> page is assigned to a processor and then that processor takes objects off
> the freelist and may free objects back to the freelist of that page that
> was assigned to a processor. Memory wastage may only occur because
> each processor needs to have a separate page from which to allocate. SLAB
> like designs needs to put a large number of objects in queues which may
> keep a number of pages in the allocated pages pool although all objects
> are unused. That does not occur with slub.

That's wrong. SLUB keeps completely free pages on its partial lists, and
also IIRC can keep free pages pinned in the per-cpu page. I have actually
seen SLQB use less memory than SLUB in some situations for this reason.


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