Messages in this thread | | | Date | Fri, 23 Sep 2005 07:14:24 +0200 | From | Eric Dumazet <> | Subject | Re: [PATCH 2/3] netfilter : 3 patches to boost ip_tables performance |
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Willy Tarreau a écrit : > On Thu, Sep 22, 2005 at 03:05:50PM +0200, Eric Dumazet wrote: > (...) > >>It was necessary to get the best code with gcc-3.4.4 on i386 and >>gcc-4.0.1 on x86_64 >> >>For example : >> >>bool1 = FWINV(ret != 0, IPT_INV_VIA_OUT); >>if (bool1) { >> >>gives a better code than : >> >>if (FWINV(ret != 0, IPT_INV_VIA_OUT)) { >> >>(one less conditional branch) >> >>Dont ask me why, it is shocking but true :( > > > I also noticed many times that gcc's optimization of "if (complex condition)" > is rather poor and it's often better to put it in a variable before. I even > remember that if you use an intermediate variable, it can often generate a > CMOV instruction on processors which support it, while it produces cond tests > and jumps without the variable. Generally speaking, if you want fast code, > you have to write it as a long sequence of small instructions, just as if > you were writing assembly. As you said, shocking but true.
Even without CMOV support, the suggested patch helps :
Here is the code generated with gcc-3.4.4 on a pentium4 (i686) for :
/********************/ bool1 = ((ip->saddr&ipinfo->smsk.s_addr) != ipinfo->src.s_addr); bool1 ^= !!(ipinfo->invflags & IPT_INV_SRCIP);
bool2 = ((ip->daddr&ipinfo->dmsk.s_addr) != ipinfo->dst.s_addr); bool2 ^= !!(ipinfo->invflags & IPT_INV_DSTIP);
if ((bool1 | bool2) != 0) {
/********************/ cb: 0f b6 56 53 movzbl 0x53(%esi),%edx cf: 8b 46 08 mov 0x8(%esi),%eax #ip->saddr d2: 23 47 0c and 0xc(%edi),%eax #ipinfo->smsk.s_addr d5: 0f b6 da movzbl %dl,%ebx d8: 3b 06 cmp (%esi),%eax #ipinfo->src.s_addr da: 88 55 cf mov %dl,0xffffffcf(%ebp) dd: 89 da mov %ebx,%edx df: 0f 95 c0 setne %al e2: c1 ea 03 shr $0x3,%edx e5: 31 c2 xor %eax,%edx e7: 8b 46 0c mov 0xc(%esi),%eax #ip->daddr&ipinfo ea: 23 47 10 and 0x10(%edi),%eax #ipinfo->dmsk.s_addr ed: 3b 46 04 cmp 0x4(%esi),%eax #ipinfo->dst.s_addr f0: 89 d8 mov %ebx,%eax f2: 0f 95 c1 setne %cl f5: c1 e8 04 shr $0x4,%eax f8: 31 c8 xor %ecx,%eax fa: 09 d0 or %edx,%eax fc: a8 01 test $0x1,%al fe: 0f 85 95 00 00 00 jne dest // only one conditional branch
As you can see the whole sequence is rather good : only one conditional branch (No CMOV instructions as you can see, so even on a i486 the code should be roughly the same)
Now here is the code generated for the original code : /********************/ if (FWINV((ip->saddr&ipinfo->smsk.s_addr) != ipinfo->src.s_addr, IPT_INV_SRCIP) || FWINV((ip->daddr&ipinfo->dmsk.s_addr) != ipinfo->dst.s_addr, IPT_INV_DSTIP)) { /********************/ cb: 0f b6 4e 53 movzbl 0x53(%esi),%ecx cf: f6 c1 08 test $0x8,%cl d2: 0f 84 af 01 00 00 je 287 <ipt_do_table+0x25d> d8: 8b 46 08 mov 0x8(%esi),%eax db: 23 47 0c and 0xc(%edi),%eax de: 3b 06 cmp (%esi),%eax e0: 0f 84 b0 01 00 00 je 296 <ipt_do_table+0x26c> e6: f6 c1 10 test $0x10,%cl e9: 0f 84 d4 01 00 00 je 2c3 <ipt_do_table+0x299> ef: 8b 46 0c mov 0xc(%esi),%eax f2: 23 47 10 and 0x10(%edi),%eax f5: 3b 46 04 cmp 0x4(%esi),%eax f8: 0f 84 98 01 00 00 je 296 <ipt_do_table+0x26c>
...
287: 8b 46 08 mov 0x8(%esi),%eax 28a: 23 47 0c and 0xc(%edi),%eax 28d: 3b 06 cmp (%esi),%eax 28f: 2e 0f 84 50 fe ff ff je,pn e6 <ipt_do_table+0xbc> 296: 0f b7 46 5a movzwl 0x5a(%esi),%eax 29a: 01 c6 add %eax,%esi 29c: 8b 4d f0 mov 0xfffffff0(%ebp),%ecx 29f: 85 c9 test %ecx,%ecx 2a1: 0f 84 24 fe ff ff je cb <ipt_do_table+0xa1>
...
2c3: 8b 46 0c mov 0xc(%esi),%eax 2c6: 23 47 10 and 0x10(%edi),%eax 2c9: 3b 46 04 cmp 0x4(%esi),%eax 2cc: 75 c8 jne 296 <ipt_do_table+0x26c> 2ce: e9 2b fe ff ff jmp fe <ipt_do_table+0xd4>
/******************/ As you can see, that a lot of conditional branches, that cannot be predicted correctly by the cpu, unless consecutives iptables rules generate the same flow.
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