[SPDK] Re: A question for bdev_io cache

[Yijing Wang <wangyijing0307 at gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 1730 bytes --]

Hi Jim, thanks for your information, now I understand why we use spdk 
bdev owner cache.

Thanks!

Yijing.


在 2019/10/9 下午12:06, Harris, James R 写道:
> Hi Yijing,
>
> We used to use a DPDK rte_mempool for the spdk_bdev_io structures.  Here's the commit where we changed it:
>
> https://github.com/spdk/spdk/commit/534d9c20020018aa2eac0a113987f6ba04855b5a
>
> The primary problem we had with the rte_mempool cache is that it only works on DPDK threads.  So applications that use the SPDK bdev layer without using DPDK threads would incur massive contention on the rte_mempool global lock.  This was especially evident with the SPDK bdev fio_plugin, which was specifically designed to show how to use the SPDK bdev layer without using DPDK threads (or the rest of the SPDK event framework).
>
> -Jim
>
> 	
> On 10/8/19, 7:09 PM, "Yijing Wang" <wangyijing0307(a)gmail.com> wrote:
>
>      Hi James,
>      
>            I'm a spdk developer from china, I'm sorry to disturb you,
>      recently I read the latest spdk bdev io code, and found that spdk
>      prepopulate per-thread bdev_io cache in bdev module.
>      
>      As we knew, dpdk rte_mempool_create has a parameter cache_size which
>      allow users to cache some objects in local cpu,  my question is why you
>      post the patch to create bdev io cache
>      
>      in bdev own list, but not use default dpdk rte_mempool cache mechnasim？
>      The reason is dpdk rte_mempool cache cannot prevent a bdev device in a
>      thread from starvation? Since dpdk
>      
>      rte_mempool cache is preserved for all users in the same cpu thread.
>      
>      
>      Thanks!
>      
>      Yijing.
>      
>