Re: [PATCH 1/1] network memory allocator.

From: Evgeniy Polyakov
Date: Tue Aug 15 2006 - 07:24:42 EST

On Tue, Aug 15, 2006 at 12:55:02PM +0200, Peter Zijlstra (a.p.zijlstra@xxxxxxxxx) wrote:
> > Userspace can sak for next packet and pointer to the new location will
> > be removed.
> /sak/ask/?
> I'm not understanding, if you have a page A with two packets, a and b;
> once you map that page into user-space that process has access to both
> packets, which is a security problem. How are you going to solve this?

Yep, there is such issue.
But no one is ever going to replace socket code with zero-copy
interfaces - Linux has backward compatibility noone ever had, so
send()/recv() will be there.
It is new interface which can be changed as described in previous
e-mails - copy if next chunk belongs to different socket and so on.

Initial user will be sniffer, which should get all packets.

> Also note that zero-copy sending does not have this problem, since data
> is already grouped by socket.
> > > > As described in recent threads [3] it is also possible to eliminate any
> > > > kind of main system OOM influence on network dataflow processing, thus
> > > > it is possible to prevent deadlock for systems, which use network as
> > > > memory storage (swap over network, iSCSI, NBD and so on).
> > >
> > > How? You have never stated how you will avoid getting all packets stuck
> > > in blocked sockets.
> >
> > Each socket has it's limit, so if allocator got enough memory, blocked
> > sockets will not affect it's behaviour.
> But isn't the total capacity of the network stack much larger than any
> allocator can provide?

TCP has 768kb limit on my amd64 with 1gb of ram, so I expect allocator
can handle all requests.
And there is a simple task in TODO list to dynamically grow cache when
threshold of memory is in use. It is really simple task and will be
implemented as soon as I complete suggestions mentioned by Andrew Morton.

> > > On another note, I think you misunderstand our SLAB allocator; we do not
> > > round up to nearest order page alloc per object; SLAB is build to avoid
> > > that and is designed to pack equal size objects into pages. The kmalloc
> > > allocator is build on top of several SLAB allocators; each with its
> > > specific size objects to serve.
> > >
> > > For example, the 64 byte SLAB will serve 64 byte objects, and packs
> > > about PAGE_SIZE/64 per page (about since there is some overhead).
> > >
> > > So the actual internal fragmentation of the current kmalloc/SLAB
> > > allocator is not as bad as you paint it. The biggest problem we have
> > > with the SLAB thing is getting pages back from it. (And the horrific
> > > complexity of the current implementation)
> >
> > Ok, not SLAB, but kmaloc/SLAB.
> The page-allocator does what you describe, but hardly anybody uses that
> to store small objects.

Network stack uses kmalloc.

> Page allocator - buddy allocator, gives out memory in 1<<n pages.
> SLAB allocator - uses the page allocator for backing, each SLAB issues
> objects of a fixed, predetermined size, packed in pages.
> kmalloc - uses a collection of SLAB allocators to issue 'variable' size
> objects (see kmalloc_sizes.h - as you will see internal fragmentation
> can become quite large for larger objects, but small objects do rather
> well - and one could always add a frequently used size if it shows to be
> beneficial).

There is no "frequently used size", kmalloc() does not know what size is
frequent and what is not. And there are other mentioned problems with
kmalloc/SLAB besides power-of-two, which prevent fragmentation problem

> > That allocator uses power-of-two allocation, so there is extremely
> > large overhead for several (and in some cases for all) usage cases
> > (e1000 with jumbo frames and unix sockets).
> Wrong example :-), e1000 is the only driver that doesn't do high order
> allocs for jumbo frames. But yes, the other drivers should be fixed,
> relying on higher order allocations is unsound.

:) do you read netdev@? There are several quite long recent discussions
where network hackers blame exactly e1000 for it's hardware problems and
ugly memory usage model.
We even think how to change struct sk_buff - Holy Grail of network code
- just to help e1000 driver (well, not exactly for e1000, but that
driver was a cause).

> > SLAB allows to have chunks of memory from differenct CPU, so it is
> > impossible to create defragmentation, thus kmalloc/SLAB by design will
> > suffer from fragmentation.
> *confused* memory is not bound to CPUs other than by NUMA, but even
> there there is only a single address space.

Each slab can have objects allocated in different CPUs, it was done to
reduce freeing algorithm. If system wants to defragment several objects
into bigger one, it must check all CPUs and find in which cache those
objects are placed, which is extremely expensive, so SLAB can not
perform defragmentation.

> > Graphs of power-of-two vs. NTA overhead is shown on projects' homepage
> > - overhead is extremely large.
> Yes seen that, but as stated, hardly anybody uses the page allocator to
> store small objects. However if you do, you get large internal
> fragmentation but zero external fragmentation (on that allocation
> level).

Truncated cat /proc/slabinfo on my machine (usual desktop):
size-32 1170 1232 32
size-128 663 780 128
size-64 4239 9558 64

> This is where my SROG allocator comes in, it is used to group objects by
> lifetime and returns the pages to the page allocator. This makes the
> whole allocator short-lived and hence cannot add (external)
> fragmentation on this level. The use I have for that is that I can then
> properly gauge how much memory there is available. External
> fragmentation and guarantees can be difficult to reconcile.
> I have no idea how fast/slow the SROG allocator is, and don't really
> care since its only used as a fallback allocator; what I do care about
> is determinism (in space).
> However, I do have a patch that converts the whole skb layer to use the
> SROG allocator, not only the payload, so I could do some test. But this
> is not a serious candidate until all jumbo frame capable drivers have
> been converted to skb fragments instead of high order allocations - a
> Good Thing [tm].

You created SROG after my suggestion and discussion about NTA and it works
well for it's purpose (doesn't it?), further extension could lead to creation
of NTA (or could not).
SROG is a wrapper on top of alloc_pages and list of free objects,
there are "several" differencies between allocators and I do not see how
they can compete right now.

Evgeniy Polyakov
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