On Fri, 17 Mar 2000 00:21:15 +0100, you wrote:
>Hi,
>
>The pid hash function in include/linux/sched.h looks like this:
>
>#define pid_hashfn(x) ((((x) >> 8) ^ (x)) & (PIDHASH_SZ - 1))
>
>What is the point of the >> 8 ? Since pids are assigned linearly (more
>or less), wouldn't ((x) & (PIDHASH_SZ - 1)) be just as good?
You are overlooking that XOR operator, I think?
Without the ((x)>>8), an 8 bit hash would map PIDs 2 and 258 to 2;
with it, 2 maps to 2, 258 maps to 3.
Not a huge difference, perhaps, but it does make the hash less linear
than the PIDs, at least.
James.
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