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Re: [Qemu-devel] [PATCH 06/10] include: add xxhash.h
From: |
Emilio G. Cota |
Subject: |
Re: [Qemu-devel] [PATCH 06/10] include: add xxhash.h |
Date: |
Wed, 6 Apr 2016 18:59:29 -0400 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Wed, Apr 06, 2016 at 12:39:42 +0100, Alex Bennée wrote:
>
> Emilio G. Cota <address@hidden> writes:
>
> > xxhash is a fast, high-quality hashing function. The appended
> > brings in the 32-bit version of it, with the small modification that
> > it assumes the data to be hashed is made of 32-bit chunks; this increases
> > speed slightly for the use-case we care about, i.e. tb-hash.
> >
> > The original algorithm, as well as a 64-bit implementation, can be found at:
> > https://github.com/Cyan4973/xxHash
> >
> > Signed-off-by: Emilio G. Cota <address@hidden>
> > ---
> > include/qemu/xxhash.h | 106
> > ++++++++++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 106 insertions(+)
> > create mode 100644 include/qemu/xxhash.h
> >
> > diff --git a/include/qemu/xxhash.h b/include/qemu/xxhash.h
> > new file mode 100644
> > index 0000000..a13a665
> > --- /dev/null
> > +++ b/include/qemu/xxhash.h
> > @@ -0,0 +1,106 @@
> <snip>
> > +
> > +/* u32 hash of @n contiguous chunks of u32's */
> > +static inline uint32_t qemu_xxh32(const uint32_t *p, size_t n, uint32_t
> > seed)
> > +{
>
> What is the point of seed here? I looked on the original site to see if
> there was any guidance on tuning seed but couldn't find anything. I
> appreciate the compiler can inline the constant away but perhaps we
> should #define it and drop the parameter if we are not intending to
> modify it?
The seed value would only matter if we needed consistent hashes (we don't).
Any seed value should give similar performance, given xxhash's quality.
I'll set a define for this seed if it is used in more than one place.
> Also it might be helpful to wrap the call to avoid getting the
> boilerplate sizing wrong:
>
> #define qemu_xxh32(s) qemu_xxh32_impl((const uint32_t *)s,
> sizeof(*s)/sizeof(uint32_t), 1)
>
> Then calls become a little simpler for the user:
>
> return qemu_xxh32(&k);
>
> Do we need to include a compile time check for structures that don't
> neatly divide into uint32_t chunks?
I'll give the original byte-sized function a shot and come back to this
if necessary.
Thanks,
E.
- [Qemu-devel] [PATCH 08/10] qht: QEMU's fast, resizable and scalable Hash Table, (continued)
- [Qemu-devel] [PATCH 08/10] qht: QEMU's fast, resizable and scalable Hash Table, Emilio G. Cota, 2016/04/05
- [Qemu-devel] [PATCH 01/10] translate-all: add missing fold of tb_ctx into tcg_ctx, Emilio G. Cota, 2016/04/05
- [Qemu-devel] [PATCH 09/10] qht: add test program, Emilio G. Cota, 2016/04/05
- [Qemu-devel] [PATCH 04/10] seqlock: rename write_lock/unlock to write_begin/end, Emilio G. Cota, 2016/04/05
- [Qemu-devel] [PATCH 06/10] include: add xxhash.h, Emilio G. Cota, 2016/04/05
- [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Emilio G. Cota, 2016/04/05
- Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Richard Henderson, 2016/04/05
- Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Paolo Bonzini, 2016/04/05
- Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Richard Henderson, 2016/04/05
- Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Emilio G. Cota, 2016/04/05
- Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Richard Henderson, 2016/04/05
- Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Emilio G. Cota, 2016/04/05
- Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Paolo Bonzini, 2016/04/06
- Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Emilio G. Cota, 2016/04/06
- Re: [Qemu-devel] [PATCH 07/10] tb hash: hash phys_pc, pc, and flags with xxhash, Paolo Bonzini, 2016/04/06