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Re: [Qemu-devel] [PATCH v6 10/15] qht: QEMU's fast, resizable and scalab

From: Emilio G. Cota
Subject: Re: [Qemu-devel] [PATCH v6 10/15] qht: QEMU's fast, resizable and scalable Hash Table
Date: Fri, 3 Jun 2016 05:18:28 -0400
User-agent: Mutt/1.5.23 (2014-03-12)

On Sun, May 29, 2016 at 22:52:27 +0300, Sergey Fedorov wrote:
> I was just wondering if it could be worthwhile to pass a hash function
> when initializing a QHT. Then we could have variants of qht_insert(),
> qht_remove() and qht_lookup() which does not require a computed hash
> value but call the function by themselves. This could make sense since a
> hash value passed the the functions should always be exactly the same
> for the same object.

I considered this when designing the API. I think it's not worth having
in qht; callers could have their own wrapper to do this though.

For the only caller of qht that we have so far I don't see this
as being worth the hassle.

For instance, we couldn't use the same function for lookups and
inserts/removals, since the hash function would look like:

uint32_t hash_func(void *p)
        TranslationBlock *tb = p;
        return tb_hash_func(tb->phys_pc, ...);

But for lookups we don't yet know *tb (that's what we're looking for!).
All we have is the tb_desc struct that we use for comparisons.



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