[dpdk-dev] [disscussion] mlx4 driver MLX4_PMD_TX_MP_CACHE default vaule

[Adrien Mazarguil]

Hi,

On Fri, Jul 28, 2017 at 07:58:48AM +0000, chenchanghu wrote:
> Hi,
>          When I used the mlx4 pmd, I meet a problem about MLX4_PMD_TX_MP_CACHE vaule, which is used for Memory pool to Memory region translation. The detail test is descripted below.
> 1.Test environmemt infomation:
>   a. Linux distribution: CentOS
>   b. dpdk version: dpdk-16.04
>   c. Ethernet device : mlx4 VF
>   d. pmd info: mlx4 poll-mode-driver
> 
> 2.Test diagram:
> +----------------------+    +---------------------+         +-----------------------+
> | client1       |    |  client2     |  .....    |  clientN      |
> +----------------+----+    +-------+------------+         +----------+------------+
>                   |         |                 |
>                   |         |                 |
>                +----v---------------v------------------------------v------+
>                |              share memory queue   |
>                +----------------------------+------------------------------+
>                                 |
>                                 |
>              +-------------------------------v--------------------------------+
>              |                    server            |
>              +-------------------------------+--------------------------------+
>                                 |
>                                 |
>              +-------------------------------v--------------------------------+
>              |             dpdk rte_eth_tx_burst      |
>              +-------------------------------+--------------------------------+
>                                 |
>              +-------------------------------v---------------------------------+
>              |                mlx4 pmd driver         |
>              +-----------------------------------------------------------------+
>   a. Every client has one memory pool, all clients send message to server queue in the shared memory.
>   b. Server is only one thread, and mlx4 pmd use one tx queue.
> 
> 3.Test step:
>   a. We start 30 clients, which means total mempool number reaching 30, every client will send 20 packets/second, every packet length is 10k.However,the server will do large packet segmentation before the packet send to rte_eth_tx_burst.
>   b. When we use the mlx4 pmd default MLX4_PMD_TX_MP_CACHE value which is 8, we found that the function 'rte_eth_tx_burst' cost about 40ms, which is mostly cost by the function 'ibv_reg_mr'.
>   c. Then we modify the MLX4_PMD_TX_MP_CACHE vaule to 32, which is configured the vaule 'CONFIG_RTE_LIBRTE_MLX4_TX_MP_CACHE' in the config/common_base file, we found the function 'rte_eth_tx_burst' running time is less than 5ms.

Yep, this is an old limitation that also affects the mlx5 PMD (and recently
discussed [1]).

>    Would the community modify the default CONFIG_RTE_LIBRTE_MLX4_TX_MP_CACHE value to 32 to adapt the scenario like above description, avoiding the slow operation when use too many mempool number
> which is more than the CONFIG_RTE_LIBRTE_MLX4_TX_MP_CACHE value in one tx queue.
> 
>    Please send your reply to chenchanghu at huawei.com<mailto:chenchanghu at huawei.com>, any suggestion is to be greatefully appreciated.
> 
> 4. Patch:
> diff --git a/config/common_base b/config/common_base
> index a0580d1..af6ba47 100644
> --- a/config/common_base
> +++ b/config/common_base
> @@ -207,7 +207,7 @@ CONFIG_RTE_LIBRTE_MLX4_PMD=y
> CONFIG_RTE_LIBRTE_MLX4_DEBUG=n
> CONFIG_RTE_LIBRTE_MLX4_SGE_WR_N=4
> CONFIG_RTE_LIBRTE_MLX4_MAX_INLINE=0
> -CONFIG_RTE_LIBRTE_MLX4_TX_MP_CACHE=8
> +CONFIG_RTE_LIBRTE_MLX4_TX_MP_CACHE=32
> CONFIG_RTE_LIBRTE_MLX4_SOFT_COUNTERS=1

In the past, using 8 as the default MP cache size was deemed a fair
trade-off since most applications only used a small number of
mempools. Also, recompiling DPDK after tweaking compilation options to suit
specific applications needs was not much of an issue.

Today DPDK is more library-like so setting compile-time parameters is not
always possible for applications, this is why we're in the process of
removing these options altogether as part of a major refactoring. They will
become either run-time options (through device parameters) or fully
automatic whenever possible. This should be the case for mlx4 in the 17.11
release (it's too late for 17.08).

In the meantime, considering CONFIG_RTE_LIBRTE_MLX4_PMD is disabled by
default and needs to be enabled manually, users that need a larger MP cache
need to also increase CONFIG_RTE_LIBRTE_MLX4_TX_MP_CACHE like you did.

Because updating the default value cannot possibly satisfy all use cases,
I think it's better to leave it as is for the time being in order to not
affect existing applications.

[1] http://dpdk.org/ml/archives/dev/2017-July/071405.html

-- 
Adrien Mazarguil
6WIND