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

From: Jeff Hostetler <git@jeffhostetler.com>
Date: Sep 5, 2017, 16:33 UTC
Message-ID: <9ec32edc-5aeb-53c0-7888-541f7a9db8bf@jeffhostetler.com>
In-Reply-To: <alpine.DEB.2.21.1.1709020109520.4132@virtualbox>

On 9/1/2017 7:31 PM, Johannes Schindelin wrote:

Show 29 quoted lines

> Hi Jeff,
> 
> On Wed, 30 Aug 2017, Jeff Hostetler wrote:
> 
>> 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.

I'm not sure which lines you're talking about, but I'll give it another scan and double check.

There's not much I can do about the public-inbox.org URL.

Show 20 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: ".

Good point.  Thanks.

Show 24 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.

I'm not sure what you're saying here. The iterator iterates over all entries (and handles walking collision chains), so my newly computed count should be correct and all of this is independent of the "grow-at" and table-size logic.

I'm not forcing a rehash when counting is enabled. I'm just reestablishing the expected state. The next insert may cause a rehash, but I'm not forcing it.

However, there is an assumption that the caller pre-allocated sufficient table-size space to avoid poor performance for the duration of the non-counting period.

Show 7 quoted lines

> 
> 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).

Personally, I think the 80% threshold is too aggressive (and the default size is too small), but that's a different question.

The hashmap in question contains directory pathnames, so the distribution will be completely dependent on the shape of the data.

FWIW, I created a tool to dump some of this data.  See:
     t/helper/test-lazy-init-name-hash.c

Show 25 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
>

thanks Jeff