On 14/02/2017 3:45 PM, Eric Dumazet wrote:
On Tue, Feb 14, 2017 at 4:12 AM, Jesper Dangaard Brouer
<brouer@xxxxxxxxxx> wrote:
It is important to understand that there are two cases for the cost of
an atomic op, which depend on the cache-coherency state of the
cacheline.
Measured on Skylake CPU i7-6700K CPU @ 4.00GHz
(1) Local CPU atomic op : 27 cycles(tsc) 6.776 ns
(2) Remote CPU atomic op: 260 cycles(tsc) 64.964 ns
Okay, it seems you guys really want a patch that I said was not giving
good results
Let me publish the numbers I get , adding or not the last (and not
official) patch.
If I _force_ the user space process to run on the other node,
then the results are not the ones Alex or you are expecting.
I have with this patch about 2.7 Mpps of this silly single TCP flow,
and 3.5 Mpps without it.
lpaa24:~# sar -n DEV 1 10 | grep eth0 | grep Ave
Average: eth0 2699243.20 16663.70 1354783.36 1079.95
0.00 0.00 4.50
Profile of the cpu on NUMA node 1 ( netserver consuming data ) :
54.73% [kernel] [k] copy_user_enhanced_fast_string
31.07% [kernel] [k] skb_release_data
4.24% [kernel] [k] skb_copy_datagram_iter
1.35% [kernel] [k] copy_page_to_iter
0.98% [kernel] [k] _raw_spin_lock
0.90% [kernel] [k] skb_release_head_state
0.60% [kernel] [k] tcp_transmit_skb
0.51% [kernel] [k] mlx4_en_xmit
0.33% [kernel] [k] ___cache_free
0.28% [kernel] [k] tcp_rcv_established
Profile of cpu handling mlx4 softirqs (NUMA node 0)
48.00% [kernel] [k] mlx4_en_process_rx_cq
12.92% [kernel] [k] napi_gro_frags
7.28% [kernel] [k] inet_gro_receive
7.17% [kernel] [k] tcp_gro_receive
5.10% [kernel] [k] dev_gro_receive
4.87% [kernel] [k] skb_gro_receive
2.45% [kernel] [k] mlx4_en_prepare_rx_desc
2.04% [kernel] [k] __build_skb
1.02% [kernel] [k] napi_reuse_skb.isra.95
1.01% [kernel] [k] tcp4_gro_receive
0.65% [kernel] [k] kmem_cache_alloc
0.45% [kernel] [k] _raw_spin_lock
Without the latest patch (the exact patch series v3 I submitted),
thus with this atomic_inc() in mlx4_en_process_rx_cq instead of only reads.
lpaa24:~# sar -n DEV 1 10|grep eth0|grep Ave
Average: eth0 3566768.50 25638.60 1790345.69 1663.51
0.00 0.00 4.50
Profiles of the two cpus :
74.85% [kernel] [k] copy_user_enhanced_fast_string
6.42% [kernel] [k] skb_release_data
5.65% [kernel] [k] skb_copy_datagram_iter
1.83% [kernel] [k] copy_page_to_iter
1.59% [kernel] [k] _raw_spin_lock
1.48% [kernel] [k] skb_release_head_state
0.72% [kernel] [k] tcp_transmit_skb
0.68% [kernel] [k] mlx4_en_xmit
0.43% [kernel] [k] page_frag_free
0.38% [kernel] [k] ___cache_free
0.37% [kernel] [k] tcp_established_options
0.37% [kernel] [k] __ip_local_out
37.98% [kernel] [k] mlx4_en_process_rx_cq
26.47% [kernel] [k] napi_gro_frags
7.02% [kernel] [k] inet_gro_receive
5.89% [kernel] [k] tcp_gro_receive
5.17% [kernel] [k] dev_gro_receive
4.80% [kernel] [k] skb_gro_receive
2.61% [kernel] [k] __build_skb
2.45% [kernel] [k] mlx4_en_prepare_rx_desc
1.59% [kernel] [k] napi_reuse_skb.isra.95
0.95% [kernel] [k] tcp4_gro_receive
0.51% [kernel] [k] kmem_cache_alloc
0.42% [kernel] [k] __inet_lookup_established
0.34% [kernel] [k] swiotlb_sync_single_for_cpu
So probably this will need further analysis, outside of the scope of
this patch series.
Could we now please Ack this v3 and merge it ?
Thanks.
Thanks Eric.
As the previous series caused hangs, we must run functional regression
tests over this series as well.
Run has already started, and results will be available tomorrow morning.
In general, I really like this series. The re-factorization looks more
elegant and more correct, functionally.
However, performance wise: we fear that the numbers will be drastically
lower with this transition to order-0 pages,
because of the (becoming critical) page allocator and dma operations
bottlenecks, especially on systems with costly
dma operations, such as ARM, iommu=on, etc...
We already have this exact issue in mlx5, where we moved to order-0
allocations with a fixed size cache, but that was not enough.
Customers of mlx5 have already complained about the performance
degradation, and currently this is hurting our business.
We get a clear nack from our performance regression team regarding doing
the same in mlx4.
So, the question is, can we live with this degradation until those
bottleneck challenges are addressed?
Following our perf experts feedback, I cannot just simply Ack. We need
to have a clear plan to close the perf gap or reduce the impact.
Internally, I already implemented "dynamic page-cache" and "page-reuse"
mechanisms in the driver,
and together they totally bridge the performance gap.
That's why I would like to hear from Jesper what is the status of his
page_pool API, it is promising and could totally solve these issues.
Regards,
Tariq
Notice the huge difference. And in case 2, it is enough that the remote
CPU reads the cacheline and brings it into "Shared" (MESI) state, and
the local CPU then does the atomic op.
One key ideas behind the page_pool, is that remote CPUs read/detect
refcnt==1 (Shared-state), and store the page in a small per-CPU array.
When array is full, it gets bulk returned to the shared-ptr-ring pool.
When "local" CPU need new pages, from the shared-ptr-ring it prefetchw
during it's bulk refill, to latency-hide the MESI transitions needed.
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
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