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

From: David Laight
Date: Wed Dec 10 2014 - 09:23:32 EST

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.