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    SubjectRe: [patch] arca-vm-2.2.5
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    >>>>> "CL" == Chuck Lever <cel@monkey.org> writes:

    CL> On Tue, 6 Apr 1999, Andrea Arcangeli wrote:
    >> Cool! ;)) But could you tell me _how_ do you design an hash function? Are
    >> you doing math or do you use instinct?

    CL> math. i'll post something about this soon.

    >> >but also the page hash function uses the hash table size as a shift value
    >> >when computing the index, so it may combine the interesting bits in a
    >> >different (worse) way when you change the hash table size. i'm planning
    >> >to instrument the page hash to see exactly what's going on.
    >>
    >> Agreed. This is true. I thought about that and I am resizing the hash
    >> table size to the original 11 bit now (since you are confirming that I
    >> broken the hash function).

    CL> i looked at doug's patch too, and it changes the "page_shift" value
    CL> depending on the size of the hash table. again, this *may* cause unwanted
    CL> interactions making the hash function degenerate for certain table sizes.
    CL> but i'd like to instrument the hash to watch what really happens.


    CL> i ran some simple benchmarks on our 4-way Xeon PowerEdge to see what are
    CL> the effects of your patches. here were the original patches against
    CL> 2.2.5.

    CL> the page struct alignment patch:

    >> --- linux/include/linux/mm.h Tue Mar 9 01:55:28 1999
    >> +++ mm.h Tue Apr 6 02:00:22 1999
    >> @@ -131,0 +133,6 @@
    >> +#ifdef __SMP__
    >> + /* cacheline alignment */
    >> + char dummy[(sizeof(void *) * 7 +
    >> + sizeof(unsigned long) * 2 +
    >> + sizeof(atomic_t)) % L1_CACHE_BYTES];
    >> +#endif

    Am I the only one to notice that this little bit of code is totally wrong.
    It happens to get it right for cache sizes of 16 & 32 with the current struct
    page but the code is 100% backwords.

    Eric



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