Messages in this thread | | | Date | Sat, 30 Apr 2016 10:12:38 -0700 | Subject | Re: [patch 2/7] lib/hashmod: Add modulo based hash mechanism | From | Linus Torvalds <> |
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On Sat, Apr 30, 2016 at 9:45 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote: > > I use hash_32() in net/sched/sch_fq.c, for all packets sent by Google > servers. (Note that I did _not_ use hash_ptr()) > > That's gazillions of packets per second, and the current multiply worked > just fine in term of hash spreading.
So hash_32() really is much better than hash_64(). I think we'll tweak it a bit, but largely leave it alone.
The 64-bit case needs to be tweaked a _lot_.
For the 32-bit case, I like the one that George Spelvin suggested:
#define GOLDEN_RATIO_32 0x61c88647 /* phi^2 = 1-phi */
because of his slow multiplier fallback version that we could also use:
/* Returns x * GOLDEN_RATIO_32 without a hardware multiplier */ unsigned hash_32(unsigned x) { unsigned y, z; /* Path length */ y = (x << 19) + x; /* 1 shift + 1 add */ z = (x << 9) + y; /* 1 shift + 2 add */ x = (x << 23) + z; /* 1 shift + 3 add */ z = (z << 8) + y; /* 2 shift + 3 add */ x = (x << 6) - x; /* 2 shift + 4 add */ return (z << 3) + x; /* 3 shift + 4 add */ }
and I don't think that we really need the several big constants with the fancy "full cascade" function.
If you have a test-case for that sch_fq.c case, it might be a good idea to test the above GOLDEN_RATIO_32 value, but quite frankly, I don't see any way it would be materially different from the one we use now. It does avoid that long series of zeroes in the low bits, but that's actually not a huge problem for the 32-bit hash to begin with. It's not nearly as long a series (or in the wrong bit positions) as the 64-bit hash multiplier value had.
Also, I suspect that since you hash the kernel "struct sock" pointers, you actually never get the kinds of really bad patterns that Thomas had.
But maybe you use hash_32() on a pointer because you noticed that hash_long() or hash_ptr() (which use hash_64 on 64-bit architectures, and would have been more natural) gave worse performance?
Maybe you thought that it was the bigger multiply that caused the performance problems? If you did performance work, I suspect it really could have been that hash_64() did a bad job for you.
Linus
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