Re: [PATCH] mempool: limit cache_size

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

On 18/05/15 15:13, Ananyev, Konstantin wrote:
>
>
>> -----Original Message-----
>> From: Zoltan Kiss [mailto:zoltan.kiss@linaro.org]
>> Sent: Monday, May 18, 2015 2:31 PM
>> To: Ananyev, Konstantin; dev@dpdk.org
>> Subject: Re: [dpdk-dev] [PATCH] mempool: limit cache_size
>>
>>
>>
>> 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)
>
> That's quite expensive and I think would slow down mempool_put() quite a lot .
> So I'd suggest we keep cache_flushthresh as it is.
Ok, I have posted a v2 based on your suggestion.
>
>>
>> Furthermore, do we want to expose the flush threshold multiplier through
>> the config file?
>
> Hmm, my opinion is no - so far no one ask for that,
> and as general tendency - we trying to reduce number of options in config file.
> Do you have any good justification when current value is not good enough?

Nothing special, just the arbitrary value choice seemed a bit odd.
> Anyway, that probably could be a subject of another patch/discussion.
> Konstantin
>
>>
>>> ?
>>>
>>> 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
>>>>>>>