Subject: Re: Optimizing instruction-cache, more packets at each stage

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 22 Jan 2016 13:33:41 +0100

On Thu, 21 Jan 2016 09:48:36 -0800
Eric Dumazet <[email-masked]> wrote:

> On Thu, 2016-01-21 at 08:38 -0800, Tom Herbert wrote:
> 
> > Sure, but the receive path is parallelized.  
> 
> This is true for multiqueue processing, assuming you can dedicate many
> cores to process RX.
> 
> >  Improving parallelism has
> > continuously shown to have much more impact than attempting to
> > optimize for cache misses. The primary goal is not to drive 100Gbps
> > with 64 packets from a single CPU. It is one benchmark of many we
> > should look at to measure efficiency of the data path, but I've yet to
> > see any real workload that requires that...
> > 
> > Regardless of anything, we need to load packet headers into CPU cache
> > to do protocol processing. I'm not sure I see how trying to defer that
> > as long as possible helps except in cases where the packet is crossing
> > CPU cache boundaries and can eliminate cache misses completely (not
> > just move them around from one function to another).  
> 
> Note that some user space use multiple core (or hyper threads) to
> implement a pipeline, using a single RX queue.
> 
> One thread can handle one stage (device RX drain) and prefetch data into
> shared L1/L2 (and/or shared L3 for pipelines with more than 2 threads)
> 
> The second thread process packets with headers already in L1/L2

I agree. I've heard experiences where DPDK users use 2 core for RX, and
1 core for TX, and achieve 10G wirespeed (14Mpps) real IPv4 forwarding
with full Internet routing table look up.

One of the ideas behind my alf_queue, is that it can be used for
efficiently distributing object (pointers) between threads.
1. because it only transfers the pointers (not touching object), and
2. because it enqueue/dequeue multiple objects with a single locked cmpxchg.
Thus, lower in the message passing cost between threads.

> This way, the ~100 ns (or even more if you also consider skb
> allocations) penalty to bring packet headers do not hurt PPS.

I've studied the allocation cost in great detail, thus let me share my
numbers, 100 ns is too high:

Total cost of alloc+free for 256 byte objects (on CPU i7-4790K @ 4.00GHz).
The cycles count should be comparable with other CPUs, but that nanosec
measurement is affected by the very high clock freq of this CPU.

Kmem_cache fastpath "recycle" case:
 SLUB => 44 cycles(tsc) 11.205 ns
 SLAB => 96 cycles(tsc) 24.119 ns.

The problem is that real use-cases in the network stack, almost always
hit the slowpath in kmem_cache allocators.

Kmem_cache "slowpath" case:
 SLUB => 117 cycles(tsc) 29.276 ns
 SLAB => 101 cycles(tsc) 25.342 ns

I've addressed this "slowpath" problem in the SLUB and SLAB allocators,
by introducing a bulk API, which amortize the needed sync-mechanisms.

Kmem_cache using bulk API:
 SLUB => 37 cycles(tsc) 9.280 ns
 SLAB => 20 cycles(tsc) 5.035 ns

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer