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    SubjectRe: [patch v2] epoll use a single inode ...
    Date
    On Tuesday 06 March 2007 18:28, Eric Dumazet wrote:
    > On Tuesday 06 March 2007 18:19, Linus Torvalds wrote:
    >
    > > > Using reciprocal divides permits to change each divide by two
    > > > multiplies, less expensive on current CPUS.
    > >
    > > Are you sure?
    >
    > I am going to test this, but at least on Opterons, the reciprocal divide I
    > added into mm/slab.c gave me a nice speedup.
    >

    With attached test program (one million calls to pipe()), I got about 0.1 s of
    speedup on my 1.6 GHz Pentium(M) dell D610 machine
    (3.350 s instead of 3.450 s, on many runs)

    Thats about 100 ns saved per number() call

    But then I realized that on ia32, gcc compilers is not very smart :

    static inline u32 reciprocal_divide(u32 A, u32 R)
    {
    return (u32)(((u64)A * R) >> 32);
    }

    It generates two multiplies... arg...

    //begin of reciprocal_divide()
    4b0: 8b 4c 24 28 mov 0x28(%esp),%ecx
    4b4: 89 f0 mov %esi,%eax
    4b6: f7 64 24 24 mull 0x24(%esp)
    4ba: 0f af ce imul %esi,%ecx
    4bd: 8d 14 11 lea (%ecx,%edx,1),%edx
    // end of reciprocal_divide()
    4c0: 8b 8c 24 8c 00 00 00 mov 0x8c(%esp),%ecx
    4c7: 89 d0 mov %edx,%eax
    4c9: 0f af c8 imul %eax,%ecx
    4cc: 29 ce sub %ecx,%esi
    4ce: 8b 4c 24 1c mov 0x1c(%esp),%ecx
    4d2: 0f b6 34 31 movzbl (%ecx,%esi,1),%esi
    4d6: 89 f1 mov %esi,%ecx
    4d8: 89 c6 mov %eax,%esi
    4da: 88 0f mov %cl,(%edi)
    4dc: 47 inc %edi
    4dd: ff 44 24 20 incl 0x20(%esp)
    4e1: 85 c0 test %eax,%eax
    4e3: 75 cb jne 4b0 <number+0x160>

    So even with a total of 3 multiplies per digit, we win...

    Maybe some bit of x86 asm is needed to make gcc be smarter (using only one
    multiply for reciprocal_divide())

    /*
    * micro benchmark to time calls to pipe()/close()
    */
    main()
    {
    int fd[100*2];
    unsigned int l, i;

    for (l = 0 ; l < 10000 ; l++) {
    for (i = 0 ; i < 100*2 ; i+=2)
    pipe(fd + i);
    for (i = 0 ; i < 100*2 ; i++)
    close(fd[i]);
    }
    }
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