On Mon, Aug 14, 2006 at 02:25:13PM +0200, Peter Zijlstra (a.p.zijlstra@chello.nl) wrote:

On Mon, 2006-08-14 at 15:04 +0400, Evgeniy Polyakov wrote:

quoted

Defragmentation is a part of freeing algorithm and initial fragmentation
avoidance is being done at allocation time by removing power-of-two
allocations. Rate of fragmentation can be found in some userspace
modlling tests being done for both power-of-two SLAB-like and NTA
allocators. (more details on project's homepage [4]).

Only with a perfect allocation pattern. And then still only internal
fragmentation; your allocator is still vulnerable to external
fragmentation - you cannot move allocated chunks around because there
are pointers into it, hence you will suffer from external fragmentation.

http://en.wikipedia.org/wiki/Fragmentation_%28computer%29

Nature of network dataflow does not obey to that requirements - all
chunks are "short-lives", i.e. sooner or later they will be put back and
thus initial region will be repaired.
With existing allocator it never happens by deisgn.

quoted

Benchmarks with trivial epoll based web server showed noticeble (more
than 40%) imrovements of the request rates (1600-1800 requests per
second vs. more than 2300 ones). It can be described by more
cache-friendly freeing algorithm, by tighter objects packing and thus
reduced cache line ping-pongs, reduced lookups into higher-layer caches
and so on.

Nice :-)

quoted

Design of allocator allows to map all node's pages into userspace thus
allows to have true zero-copy support for both sending and receiving
dataflows.

I'm still not clear on how you want to do this, only the trivial case of
a sniffer was mentioned by you. To be able to do true zero-copy receive
each packet will have to have its own page(s). Simply because you do not
know the destination before you receive it, the packet could end up
going to a whole different socket that the prev/next. As soon as you
start packing multiple packets on 1 page, you've lost the zero-copy
receive game.

Userspace can sak for next packet and pointer to the new location will
be removed.

quoted

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.
 

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.
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).
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.
Graphs of power-of-two vs. NTA overhead is shown on projects' homepage 
- overhead is extremely large.

-- 
	Evgeniy Polyakov