Re: [PATCH] hashmap: add API to disable item counting when threaded

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Sep 1, 2017, 23:31 UTC
Message-ID: <alpine.DEB.2.21.1.1709020109520.4132@virtualbox>
In-Reply-To: <20170830185922.10107-2-git@jeffhostetler.com>

Hi Jeff,

On Wed, 30 Aug 2017, Jeff Hostetler wrote:

Show 20 quoted lines

> From: Jeff Hostetler <jeffhost@microsoft.com>
> 
> This is to address concerns raised by ThreadSanitizer on the mailing
> list about threaded unprotected R/W access to map.size with my previous
> "disallow rehash" change (0607e10009ee4e37cb49b4cec8d28a9dda1656a4).
> See:
> https://public-inbox.org/git/adb37b70139fd1e2bac18bfd22c8b96683ae18eb.1502780344.git.martin.agren@gmail.com/
> 
> Add API to hashmap to disable item counting and to disable automatic
> rehashing.  Also include APIs to re-enable item counting and automatica
> rehashing.
> 
> When item counting is disabled, the map.size field is invalid.  So to
> prevent accidents, the field has been renamed and an accessor function
> hashmap_get_size() has been added.  All direct references to this field
> have been been updated.  And the name of the field changed to
> map.private_size to communicate thie.
> 
> Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
> ---

The Git contribution process forces me to point out lines longer than 80 columns. I wish there was already an automated tool to fix that, but we (as in "the core Git developers") have not yet managed to agree on one. So I'll have to ask you to identify and fix them manually.

Show 12 quoted lines

> @@ -253,6 +253,19 @@ static inline void hashmap_entry_init(void *entry, unsigned int hash)
>  }
>  
>  /*
> + * Return the number of items in the map.
> + */
> +inline unsigned int hashmap_get_size(struct hashmap *map)
> +{
> +	if (map->do_count_items)
> +		return map->private_size;
> +
> +	/* TODO Consider counting them and returning that. */

I'd rather not. If counting is disabled, it is disabled.

> +	die("hashmap_get_size: size not set");

Before anybody can ask for this message to be wrapped in _(...) to be translateable, let me suggest instead to add the prefix "BUG: ".

Show 13 quoted lines

> +static inline void hashmap_enable_item_counting(struct hashmap *map)
> +{
> +	void *item;
> +	unsigned int n = 0;
> +	struct hashmap_iter iter;
> +
> +	hashmap_iter_init(map, &iter);
> +	while ((item = hashmap_iter_next(&iter)))
> +		n++;
> +
> +	map->do_count_items = 1;
> +	map->private_size = n;
> +}

BTW this made me think that we may have a problem in our code since switching from my original hashmap implementation to the bucket one added in 6a364ced497 (add a hashtable implementation that supports O(1) removal, 2013-11-14): while it is not expected that there are many collisions, the "grow_at" logic still essentially assumes the number of buckets to be equal to the number of hashmap entries.

Your code simply reiterates that assumption, so I do not blame you for anything here, nor ask you to change your patch.

But it does look a bit weird to assume so much about the nature of our data, without having any real-life numbers. I wish I had more time so that I could afford to run a couple of tests on this hashmap, such as: what is the typical difference between bucket count and entry count, or the median of the bucket sizes when the map is 80% full (i.e. *just* below the grow threshold).

Show 15 quoted lines

> diff --git a/name-hash.c b/name-hash.c
> index 0e10f3e..829ff59 100644
> --- a/name-hash.c
> +++ b/name-hash.c
> @@ -580,9 +580,11 @@ static void lazy_init_name_hash(struct index_state *istate)
>  			NULL, istate->cache_nr);
>  
>  	if (lookup_lazy_params(istate)) {
> -		hashmap_disallow_rehash(&istate->dir_hash, 1);
> +		hashmap_disable_item_counting(&istate->dir_hash);
> +		hashmap_disable_auto_rehash(&istate->dir_hash);
>  		threaded_lazy_init_name_hash(istate);
> -		hashmap_disallow_rehash(&istate->dir_hash, 0);
> +		hashmap_enable_auto_rehash(&istate->dir_hash);
> +		hashmap_enable_item_counting(&istate->dir_hash);

By your rationale, it would be enough to simply disable and re-enable counting...

The rest of the patch looks just dandy to me.

Thanks, Dscho