Re: [RFC PATCH 0/3] Faster than SLAB caching of SKBs with qmempool (backed by alf_queue)

From: Jesper Dangaard Brouer
Date: Wed Dec 10 2014 - 09:40:53 EST

On Wed, 10 Dec 2014 14:22:22 +0000
David Laight <David.Laight@xxxxxxxxxx> wrote:

> From: Jesper Dangaard Brouer
> > The network stack have some use-cases that puts some extreme demands
> > on the memory allocator. One use-case, 10Gbit/s wirespeed at smallest
> > packet size[1], requires handling a packet every 67.2 ns (nanosec).
> >
> > Micro benchmarking[2] the SLUB allocator (with skb size 256bytes
> > elements), show "fast-path" instant reuse only costs 19 ns, but a
> > closer to network usage pattern show the cost rise to 45 ns.
> >
> > This patchset introduce a quick mempool (qmempool), which when used
> > in-front of the SKB (sk_buff) kmem_cache, saves 12 ns on "fast-path"
> > drop in iptables "raw" table, but more importantly saves 40 ns with
> > IP-forwarding, which were hitting the slower SLUB use-case.
> >
> >
> > One of the building blocks for achieving this speedup is a cmpxchg
> > based Lock-Free queue that supports bulking, named alf_queue for
> > Array-based Lock-Free queue. By bulking elements (pointers) from the
> > queue, the cost of the cmpxchg (approx 8 ns) is amortized over several
> > elements.
> It seems to me that these improvements could be added to the
> underlying allocator itself.
> Nesting allocators doesn't really seem right to me.

Yes, I would very much like to see these ideas integrated into the
underlying allocators (hence addressing the mm-list).

This patchset demonstrates that it is possible to do something faster
than the existing SLUB allocator. Which the network stack have a need

Best regards,
Jesper Dangaard Brouer
MSc.CS, Sr. Network Kernel Developer at Red Hat
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