Re: [PATCH] mempool: limit cache_size

[Zoltan Kiss <zoltan.kiss@linaro.org>]

On 18/05/15 14:14, Ananyev, Konstantin wrote:
>
>
>> -----Original Message-----
>> From: Zoltan Kiss [mailto:zoltan.kiss@linaro.org]
>> Sent: Monday, May 18, 2015 1:50 PM
>> To: Ananyev, Konstantin; dev@dpdk.org
>> Subject: Re: [dpdk-dev] [PATCH] mempool: limit cache_size
>>
>>
>>
>> On 18/05/15 13:41, Ananyev, Konstantin wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Zoltan Kiss
>>>> Sent: Monday, May 18, 2015 1:28 PM
>>>> To: dev@dpdk.org
>>>> Subject: Re: [dpdk-dev] [PATCH] mempool: limit cache_size
>>>>
>>>> Hi,
>>>>
>>>> Any opinion on this patch?
>>>>
>>>> Regards,
>>>>
>>>> Zoltan
>>>>
>>>> On 13/05/15 19:59, Zoltan Kiss wrote:
>>>>> Otherwise cache_flushthresh can be bigger than n, and
>>>>> a consumer can starve others by keeping every element
>>>>> either in use or in the cache.
>>>>>
>>>>> Signed-off-by: Zoltan Kiss <zoltan.kiss@linaro.org>
>>>>> ---
>>>>>     lib/librte_mempool/rte_mempool.c | 3 ++-
>>>>>     lib/librte_mempool/rte_mempool.h | 2 +-
>>>>>     2 files changed, 3 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/lib/librte_mempool/rte_mempool.c b/lib/librte_mempool/rte_mempool.c
>>>>> index cf7ed76..ca6cd9c 100644
>>>>> --- a/lib/librte_mempool/rte_mempool.c
>>>>> +++ b/lib/librte_mempool/rte_mempool.c
>>>>> @@ -440,7 +440,8 @@ rte_mempool_xmem_create(const char *name, unsigned n, unsigned elt_size,
>>>>>     	mempool_list = RTE_TAILQ_CAST(rte_mempool_tailq.head, rte_mempool_list);
>>>>>
>>>>>     	/* asked cache too big */
>>>>> -	if (cache_size > RTE_MEMPOOL_CACHE_MAX_SIZE) {
>>>>> +	if (cache_size > RTE_MEMPOOL_CACHE_MAX_SIZE ||
>>>>> +	    (uint32_t) cache_size * CACHE_FLUSHTHRESH_MULTIPLIER > n) {
>>>>>     		rte_errno = EINVAL;
>>>>>     		return NULL;
>>>>>     	}
>>>
>>> Why just no 'cache_size > n' then?
>>
>> The commit message says: "Otherwise cache_flushthresh can be bigger than
>> n, and a consumer can starve others by keeping every element either in
>> use or in the cache."
>
> Ah yes, you right - your condition is more restrictive, which is better.
> Though here you implicitly convert cache_size and n to floats and compare 2 floats :
> (uint32_t) cache_size * CACHE_FLUSHTHRESH_MULTIPLIER > n)
> Shouldn't it be:
> (uint32_t)(cache_size * CACHE_FLUSHTHRESH_MULTIPLIER) > n)
> So we do conversion back to uint32_t compare to unsigned integers instead?
> Same as below:
> mp->cache_flushthresh = (uint32_t)
>                  (cache_size * CACHE_FLUSHTHRESH_MULTIPLIER);

To bring it further: how about ditching the whole cache_flushthresh 
member of the mempool structure, and use this:

#define CACHE_FLUSHTHRESH(mp) (uint32_t)((mp)->cache_size * 1.5)

Furthermore, do we want to expose the flush threshold multiplier through 
the config file?

> ?
>
> In fact, as we use it more than once, it probably makes sense to create a macro for it,
> something like:
> #define CALC_CACHE_FLUSHTHRESH(c)	((uint32_t)((c) *  CACHE_FLUSHTHRESH_MULTIPLIER)
>
> Or even
>
> #define CALC_CACHE_FLUSHTHRESH(c)	((typeof (c))((c) *  CACHE_FLUSHTHRESH_MULTIPLIER)
>
>
> Konstantin
>
>>
>>> Konstantin
>>>
>>>>> diff --git a/lib/librte_mempool/rte_mempool.h b/lib/librte_mempool/rte_mempool.h
>>>>> index 9001312..a4a9610 100644
>>>>> --- a/lib/librte_mempool/rte_mempool.h
>>>>> +++ b/lib/librte_mempool/rte_mempool.h
>>>>> @@ -468,7 +468,7 @@ typedef void (rte_mempool_ctor_t)(struct rte_mempool *, void *);
>>>>>      *   If cache_size is non-zero, the rte_mempool library will try to
>>>>>      *   limit the accesses to the common lockless pool, by maintaining a
>>>>>      *   per-lcore object cache. This argument must be lower or equal to
>>>>> - *   CONFIG_RTE_MEMPOOL_CACHE_MAX_SIZE. It is advised to choose
>>>>> + *   CONFIG_RTE_MEMPOOL_CACHE_MAX_SIZE and n / 1.5. It is advised to choose
>>>>>      *   cache_size to have "n modulo cache_size == 0": if this is
>>>>>      *   not the case, some elements will always stay in the pool and will
>>>>>      *   never be used. The access to the per-lcore table is of course
>>>>>