Re: [Qemu-devel] [PATCH v5 13/18] qht: support parallel writes

[Sergey Fedorov]

On 25/05/16 01:07, Emilio G. Cota wrote:
> On Mon, May 23, 2016 at 23:28:27 +0300, Sergey Fedorov wrote:
>> What if we turn qht::lock into a mutex and change the function as follows:
>>
>>     static inline
>>     struct qht_bucket *qht_bucket_lock__no_stale(struct qht *ht,
>>     uint32_t hash,
>>                                                  struct qht_map
>>     **pmap)       
>>     {
>>         struct qht_bucket *b;
>>         struct qht_map *map;
>>
>>         map = atomic_rcu_read(&ht->map);
>>         b = qht_map_to_bucket(map, hash);
>>
>>         qemu_spin_lock(&b->lock);
>>         /* 'ht->map' access is serialized by 'b->lock' here */
>>         if (likely(map == ht->map)) {
>>             /* no resize in progress; we're done */
>>             *pmap = map;
>>             return b;
>>         }
>>         qemu_spin_unlock(&b->lock);
>>
>>         /* resize in progress; retry grabbing 'ht->lock' */
>>         qemu_mutex_lock(&ht->lock);
>>         b = qht_map_to_bucket(ht->map, hash);
>>         *pmap = ht->map;
>>         qemu_spin_lock(&b->lock);
>>         qemu_mutex_unlock(&ht->lock);
>>
>>         return b;
>>     }
>>
>>
>> With this implementation we could:
>>  (1) get rid of qht_map::stale
>>  (2) don't waste cycles waiting for resize to complete
> I'll include this in v6.

How is it by perf?

Regards,
Sergey