Re: [RFC PATCH] nvme: request remote is usually not involved for nvme devices

Liu Song
2022-09-18T16:10:29Z

On 2022/9/18 00:50, Jens Axboe wrote:
> On 9/17/22 10:40 AM, Liu Song wrote:
>> From: Liu Song <liusong@linux.alibaba.com>
>>
>> NVMe devices usually have a 1:1 mapping between "ctx" and "hctx",
>> so when "nr_ctx" is equal to 1, there is no possibility of remote
>> request, so the corresponding process can be simplified.
> If the worry is the call overhead of blk_mq_complete_request_remote(),
> why don't we just make that available as an inline instead? That seems
> vastly superior to providing a random shortcut in a driver to avoid
> calling it.

Hi

This is what I think about it. If it is an SSD with only one hw queue,
remote requests will
appear occasionally. As a real multi-queue device, nvme usually does not
have remote
requests, so we don't need to care about it. So even if
"blk_mq_complete_request_remote"
is called, there is a high probability that it will return false, and
the cost of changing the call
to "blk_mq_complete_request_remote" to determine whether
"req->mq_hctx->nr_ctx" is 1 is
not only a function call, but more judgments in
"blk_mq_complete_request_remote".
If "blk_mq_complete_request_remote" is decorated as inline, it also
depends on the compiler,
there is uncertainty.

Thanks