Ingo Molnar a écrit :
quoted text
> * Ingo Molnar <mingo@elte.hu> wrote:
> 
>> 100.000000 total
>> ................
>>   1.469183 tcp_current_mss
> 
>                       hits (total: 146918)
>                  .........
> ffffffff804c5237:      526 <tcp_current_mss>:
> ffffffff804c5237:      526 	41 54                	push   %r12
> ffffffff804c5239:     5929 	55                   	push   %rbp
> ffffffff804c523a:       32 	53                   	push   %rbx
> ffffffff804c523b:      294 	48 89 fb             	mov    %rdi,%rbx
> ffffffff804c523e:      539 	48 83 ec 30          	sub    __PLACEHOLDER__0_x30,%rsp
> ffffffff804c5242:     2590 	85 f6                	test   %esi,%esi
> ffffffff804c5244:      444 	48 8b 4f 78          	mov    0x78(%rdi),%rcx
> ffffffff804c5248:      521 	8b af 4c 04 00 00    	mov    0x44c(%rdi),%ebp
> ffffffff804c524e:      791 	74 2a                	je     ffffffff804c527a <tcp_current_mss+0x43>
> ffffffff804c5250:      433 	8b 87 00 01 00 00    	mov    0x100(%rdi),%eax
> ffffffff804c5256:      236 	c1 e0 10             	shl    __PLACEHOLDER__0_x10,%eax
> ffffffff804c5259:      191 	89 c2                	mov    %eax,%edx
> ffffffff804c525b:      487 	23 97 fc 00 00 00    	and    0xfc(%rdi),%edx
> ffffffff804c5261:      362 	39 c2                	cmp    %eax,%edx
> ffffffff804c5263:      342 	75 15                	jne    ffffffff804c527a <tcp_current_mss+0x43>
> ffffffff804c5265:      473 	45 31 e4             	xor    %r12d,%r12d
> ffffffff804c5268:      221 	8b 87 00 04 00 00    	mov    0x400(%rdi),%eax
> ffffffff804c526e:      194 	3b 87 80 04 00 00    	cmp    0x480(%rdi),%eax
> ffffffff804c5274:      445 	41 0f 94 c4          	sete   %r12b
> ffffffff804c5278:      261 	eb 03                	jmp    ffffffff804c527d <tcp_current_mss+0x46>
> ffffffff804c527a:        0 	45 31 e4             	xor    %r12d,%r12d
> ffffffff804c527d:      185 	48 85 c9             	test   %rcx,%rcx
> ffffffff804c5280:      686 	74 15                	je     ffffffff804c5297 <tcp_current_mss+0x60>
> ffffffff804c5282:     1806 	8b 71 7c             	mov    0x7c(%rcx),%esi
> ffffffff804c5285:        1 	3b b3 5c 03 00 00    	cmp    0x35c(%rbx),%esi
> ffffffff804c528b:       21 	74 0a                	je     ffffffff804c5297 <tcp_current_mss+0x60>
> ffffffff804c528d:        0 	48 89 df             	mov    %rbx,%rdi
> ffffffff804c5290:        0 	e8 8b fb ff ff       	callq  ffffffff804c4e20 <tcp_sync_mss>
> ffffffff804c5295:        0 	89 c5                	mov    %eax,%ebp
> ffffffff804c5297:      864 	48 8d 4c 24 28       	lea    0x28(%rsp),%rcx
> ffffffff804c529c:      634 	48 8d 54 24 10       	lea    0x10(%rsp),%rdx
> ffffffff804c52a1:      995 	31 f6                	xor    %esi,%esi
> ffffffff804c52a3:        0 	48 89 df             	mov    %rbx,%rdi
> ffffffff804c52a6:        2 	e8 f2 fe ff ff       	callq  ffffffff804c519d <tcp_established_options>
> ffffffff804c52ab:      859 	8b 8b e8 03 00 00    	mov    0x3e8(%rbx),%ecx
> ffffffff804c52b1:      936 	83 c0 14             	add    __PLACEHOLDER__0_x14,%eax
> ffffffff804c52b4:        6 	0f b7 d1             	movzwl %cx,%edx
> ffffffff804c52b7:        0 	39 d0                	cmp    %edx,%eax
> ffffffff804c52b9:      911 	74 04                	je     ffffffff804c52bf <tcp_current_mss+0x88>
> ffffffff804c52bb:        0 	29 d0                	sub    %edx,%eax
> ffffffff804c52bd:        0 	29 c5                	sub    %eax,%ebp
> ffffffff804c52bf:        0 	45 85 e4             	test   %r12d,%r12d
> ffffffff804c52c2:     6894 	89 e8                	mov    %ebp,%eax
> ffffffff804c52c4:        0 	74 38                	je     ffffffff804c52fe <tcp_current_mss+0xc7>
> ffffffff804c52c6:      990 	48 8b 83 68 03 00 00 	mov    0x368(%rbx),%rax
> ffffffff804c52cd:      642 	8b b3 04 01 00 00    	mov    0x104(%rbx),%esi
> ffffffff804c52d3:        3 	48 89 df             	mov    %rbx,%rdi
> ffffffff804c52d6:      240 	66 2b 70 30          	sub    0x30(%rax),%si
> ffffffff804c52da:      588 	66 2b b3 7e 03 00 00 	sub    0x37e(%rbx),%si
> ffffffff804c52e1:        2 	66 29 ce             	sub    %cx,%si
> ffffffff804c52e4:      284 	ff ce                	dec    %esi
> ffffffff804c52e6:      664 	0f b7 f6             	movzwl %si,%esi
> ffffffff804c52e9:        2 	e8 0a fb ff ff       	callq  ffffffff804c4df8 <tcp_bound_to_half_wnd>
> ffffffff804c52ee:       68 	0f b7 d0             	movzwl %ax,%edx
> ffffffff804c52f1:     1870 	89 c1                	mov    %eax,%ecx
> ffffffff804c52f3:        0 	89 d0                	mov    %edx,%eax
> ffffffff804c52f5:        0 	31 d2                	xor    %edx,%edx
> ffffffff804c52f7:     2135 	f7 f5                	div    %ebp
> ffffffff804c52f9:   107010 	89 c8                	mov    %ecx,%eax
> ffffffff804c52fb:     1670 	66 29 d0             	sub    %dx,%ax
> ffffffff804c52fe:        0 	66 89 83 ea 03 00 00 	mov    %ax,0x3ea(%rbx)
> ffffffff804c5305:        4 	48 83 c4 30          	add    __PLACEHOLDER__0_x30,%rsp
> ffffffff804c5309:      855 	89 e8                	mov    %ebp,%eax
> ffffffff804c530b:        0 	5b                   	pop    %rbx
> ffffffff804c530c:      797 	5d                   	pop    %rbp
> ffffffff804c530d:        0 	41 5c                	pop    %r12
> ffffffff804c530f:        0 	c3                   	retq   
> 
> apparently this division causes 1.0% of tbench overhead:
> 
> ffffffff804c52f5:        0 	31 d2                	xor    %edx,%edx
> ffffffff804c52f7:     2135 	f7 f5                	div    %ebp
> ffffffff804c52f9:   107010 	89 c8                	mov    %ecx,%eax
> 
> (gdb) list *0xffffffff804c52f7
> 0xffffffff804c52f7 is in tcp_current_mss (net/ipv4/tcp_output.c:1078).
> 1073					  inet_csk(sk)->icsk_af_ops->net_header_len -
> 1074					  inet_csk(sk)->icsk_ext_hdr_len -
> 1075					  tp->tcp_header_len);
> 1076	
> 1077			xmit_size_goal = tcp_bound_to_half_wnd(tp, xmit_size_goal);
> 1078			xmit_size_goal -= (xmit_size_goal % mss_now);
> 1079		}
> 1080		tp->xmit_size_goal = xmit_size_goal;
> 1081	
> 1082		return mss_now;
> (gdb) 
> 
> it's this division:
> 
>         if (doing_tso) {
>         [...]
> 			xmit_size_goal -= (xmit_size_goal % mss_now);
> 
> Has no-one hit this before? Perhaps this is why switching loopback 
> networking to TSO had a performance impact for others?

Yes, I mentioned it later. But apparently you dont read my mails, so
I will just stop now.

quoted text
> 
> It's still a bit weird ... how can a single division cause this much 
> overhead? tcp_bound_to_half_wnd() [which is called straight before 
> this sequence] seems low-overhead.
> 
> 	Ingo
> 
> 


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