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Re: [PATCH 4/7] libihash: use fast binary scaling to determine the load


From: Neal H. Walfield
Subject: Re: [PATCH 4/7] libihash: use fast binary scaling to determine the load
Date: Wed, 14 May 2014 13:25:10 +0200
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At Tue, 13 May 2014 21:02:53 +0200,
Justus Winter wrote:
> diff --git a/libihash/ihash.c b/libihash/ihash.c
> index d628d75..f529a17 100644
> --- a/libihash/ihash.c
> +++ b/libihash/ihash.c
>    if (ht->size)
>      {
> -      /* Only fill the hash table up to its maximum load factor.  */
> -      if ((ht->nr_items * 100) >> __builtin_ctz (ht->size) <= ht->max_load)
> +      /* Only fill the hash table up to its maximum load factor given
> +         as "binary percent", where 128b% corresponds to 100%.  As the
> +         size is always a power of two, and 128 is also, the quotient
> +         of both is also a power of two.  Therefore, we can use bit
> +         shifts to scale the number of items.

This comment describing binary percent needs to be in the interface.

> diff --git a/libihash/ihash.h b/libihash/ihash.h
> index 8829e51..809166f 100644
> --- a/libihash/ihash.h
> +++ b/libihash/ihash.h
> @@ -79,7 +79,7 @@ struct hurd_ihash
>    /* The offset of the location pointer from the hash value.  */
>    intptr_t locp_offset;
>  
> -  /* The maximum load factor in percent.  */
> +  /* The maximum load factor in binary percent.  */
>    unsigned int max_load;

Either here.

>  
> -/* Set the maximum load factor in percent to MAX_LOAD, which should be
> -   between 50 and 100.  The default is HURD_IHASH_MAX_LOAD_DEFAULT.
> -   New elements are only added to the hash table while the number of
> -   hashed elements is that much percent of the total size of the hash
> -   table.  If more elements are added, the hash table is first
> -   expanded and reorganized.  A MAX_LOAD of 100 will always fill the
> -   whole table before enlarging it, but note that this will increase
> -   the cost of operations significantly when the table is almost full.
> +/* Set the maximum load factor in binary percent to MAX_LOAD, which
> +   should be between 64 and 128.  The default is
> +   HURD_IHASH_MAX_LOAD_DEFAULT.  New elements are only added to the
> +   hash table while the number of hashed elements is that much binary
> +   percent of the total size of the hash table.  If more elements are
> +   added, the hash table is first expanded and reorganized.  A
> +   MAX_LOAD of 128 will always fill the whole table before enlarging
> +   it, but note that this will increase the cost of operations
> +   significantly when the table is almost full.
>  
>     If the value is set to a smaller value than the current load
>     factor, the next reorganization will happen when a new item is

Or here.

Neal



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