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Date: Mon, 06 Oct 2014 05:14:50 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: David Laight <David.Laight@...LAB.COM>
Cc: Amir Vadai <amirv@...lanox.com>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
Yevgeny Petrilin <yevgenyp@...lanox.com>,
Or Gerlitz <ogerlitz@...lanox.com>,
Ido Shamay <idos@...lanox.com>
Subject: RE: [PATCH net-next] net: introduce netdevice gso_min_segs attribute
On Mon, 2014-10-06 at 10:20 +0000, David Laight wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> > Some TSO engines might have a too heavy setup cost, that impacts
> > performance on hosts sending small bursts (2 MSS per packet).
> >
> > This patch adds a device gso_min_segs, allowing drivers to set
> > a minimum segment size for TSO packets, according to the NIC
> > performance.
> >
> > Tested on a mlx4 NIC, this allows to get a ~110% increase of
> > throughput when sending 2 MSS per packet.
>
> Doesn't this all depend on what you need to optimise for.
> I can think of three^Wseveral main cases:
> 1) minimising cpu use while saturating the local network.
> 2) minimising latency for single packets.
> 3) maximising throughput for a single connection.
> 4) minimising cpu use when handling a large number of connections.
> plus all the variations in packet size.
>
> I'm not sure what you are trading for what here.
>
I am not sure you really understood.
What's the point having a 40GB NIC and not being able to reach 18 Gb on
it, even if you are willing to spend all the cpu cycles you want ?
> Maybe gso = tx_bursting is almost always faster on some hardware?
> (Especially if an idle mac engine is 'kicked' for the first packet
> of a burst.)
This has nothing to do with xmit_more.
I start 1200 flows rate limited to 3 MBytes each.
(1000 netperf -t TCP_STREAM, nothing fancy here)
Theoretical total of 3.6 GBytes per second.
Without patch :
# sar -n DEV 5 5 | grep eth0
05:07:56 AM eth0 555621.60 1111203.20 35813.03 1642923.46 0.00 0.00 0.60
05:08:01 AM eth0 555591.00 1111173.80 35810.47 1642877.52 0.00 0.00 0.40
05:08:06 AM eth0 555586.20 1111162.60 35810.06 1642861.03 0.00 0.00 0.60
05:08:11 AM eth0 555624.40 1111235.40 35812.75 1642974.19 0.00 0.00 0.60
05:08:16 AM eth0 555639.60 1111266.80 35813.21 1643017.83 0.00 0.00 0.60
Average: eth0 555612.56 1111208.36 35811.90 1642930.81 0.00 0.00 0.56
With patch :
# sar -n DEV 5 5 | grep eth0
05:07:04 AM eth0 1179478.80 2358940.40 76022.47 3487725.22 0.00 0.00 0.60
05:07:09 AM eth0 1178913.60 2357807.40 75986.60 3486044.00 0.00 0.00 0.40
05:07:14 AM eth0 1178957.40 2357897.60 75988.98 3486177.50 0.00 0.00 0.60
05:07:19 AM eth0 1177556.00 2355064.60 75899.37 3481993.37 0.00 0.00 0.60
05:07:24 AM eth0 1180321.20 2360625.20 76077.15 3490209.58 0.00 0.00 0.40
Average: eth0 1179045.40 2358067.04 75994.92 3486429.94 0.00 0.00 0.52
Ask yourself which one we prefer.
About cpu costs, we hardly see anything caused by GSO,
now we optimized __copy_skb_header()
6.36% swapper [kernel.kallsyms] [k] _raw_spin_lock
5.24% netperf [kernel.kallsyms] [k] copy_user_enhanced_fast_string
5.03% swapper [kernel.kallsyms] [k] poll_idle
3.73% swapper [kernel.kallsyms] [k] tcp_ack
2.73% swapper [kernel.kallsyms] [k] memcpy
2.49% swapper [kernel.kallsyms] [k] __skb_clone
2.41% swapper [kernel.kallsyms] [k] skb_release_data
2.33% swapper [kernel.kallsyms] [k] intel_idle
2.23% swapper [kernel.kallsyms] [k] tcp_init_tso_segs
1.99% swapper [kernel.kallsyms] [k] fq_dequeue
1.94% netperf [kernel.kallsyms] [k] tcp_sendmsg
1.82% swapper [kernel.kallsyms] [k] tcp_write_xmit
1.28% swapper [kernel.kallsyms] [k] __netif_receive_skb_core
1.23% swapper [kernel.kallsyms] [k] __copy_skb_header
1.14% swapper [kernel.kallsyms] [k] kfree
1.10% swapper [kernel.kallsyms] [k] kmem_cache_free
1.06% swapper [kernel.kallsyms] [k] mlx4_en_xmit
1.05% swapper [kernel.kallsyms] [k] tcp_wfree
1.02% swapper [kernel.kallsyms] [k] inet_gso_segment
1.01% swapper [kernel.kallsyms] [k] put_compound_page
0.98% swapper [kernel.kallsyms] [k] _raw_spin_lock_irqsave
0.96% netperf [kernel.kallsyms] [k] __alloc_skb
0.92% netperf [kernel.kallsyms] [k] _raw_spin_lock
0.89% swapper [kernel.kallsyms] [k] skb_segment
0.88% swapper [kernel.kallsyms] [k] tcp_transmit_skb
0.82% swapper [kernel.kallsyms] [k] ip_queue_xmit
0.81% swapper [kernel.kallsyms] [k] __inet_lookup_established
0.76% swapper [kernel.kallsyms] [k] __kfree_skb
0.73% swapper [kernel.kallsyms] [k] ipv4_dst_check
0.66% netperf [kernel.kallsyms] [k] tcp_ack
0.60% swapper [kernel.kallsyms] [k] __alloc_skb
0.56% swapper [kernel.kallsyms] [k] ipt_do_table
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