David Miller a écrit :
From: Eric Dumazet <[EMAIL PROTECTED]>
Date: Thu, 19 Oct 2006 07:12:58 +0200
A 66 MHz 486 can perform 1.000.000 divisions per second. Is it a 'slow' cpu ?
Sparc and some other embedded chips have no division/modulus integer
instruction and do it in software.
How many times this division will be done ? As I said, tcp session
establishment.
Are you aware a division is done in slab code when you kfree() one network
frames ? That is much more problematic than SYN packets.
So... what do you prefer :
1) Keep the modulus
2) allocate two blocks of ram (powser-of -two hash size, but one extra
indirection)
3) waste near half of ram because one block allocated, and power-of-two hash
size.
I thought the problem was that you use a modulus and non-power-of-2
hash table size because rounding up to the next power of 2 wastes
a lot of space? Given that, my suggestion is simply to not round
up to the next power-of-2, or only do so when we are very very close
to that next power-of-2.
My main problem is being able to use a large hash table on big servers.
With power-of two constraint, plus kmalloc max size constraint, we can use
half the size we could.
Are you suggesting something like :
Allocation time:
----------------
if (cpu is very_very_slow or hash size small_enough) {
ptr->size = power_of_too;
ptr->size_mask = (power_of_two - 1);
} else {
ptr->size = somevalue;
ptr->size_mask = ~0;
}
Lookup time :
---------------
if (ptr->size_mask != ~0)
slot = hash & ptr->size_mask;
else
slot = hash % ptr->size;
The extra conditional branch may be more expensive than just doing division on
99% of cpus...
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