linux-kernel - Re: [PATCH] NVMe: nvme_queue made cache friendly.

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Date:	Wed, 20 May 2015 19:04:07 +0530
From:	Parav Pandit <parav.pandit@...gotech.com>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	linux-nvme@...ts.infradead.org, axboe@...nel.dk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NVMe: nvme_queue made cache friendly.

On Wed, May 20, 2015 at 6:50 PM, Matthew Wilcox <willy@...ux.intel.com> wrote:
> On Wed, May 20, 2015 at 02:01:03PM -0400, Parav Pandit wrote:
>> nvme_queue structure made 64B cache friendly so that majority of the
>> data elements of the structure during IO and completion path can be
>> found in typical single 64B cache line size which was previously spanning beyond
>> single 64B cache line size.
>
> Have you done any performance measurements on this?

I have not done the performance test yet.

>  I find it hard to
> believe that moving q_lock to the second 64B cache line results in a
> performance improvement.

Newly aligned structure including q_lock actually fits completely in
first 64 bytes. Did I miss anything in calculation?
q_lock appears to be taken at the end of the IO processing, which
means by than sq_cmds, hctx etc fields are already accessed in same
line.
May be I should move it after q_db, instead of last element?


> Seems to me it would result in a performance
> loss, since you have to grab the lock before operating on the queue,
> and cache line prefetching tends to prefetch the _next_ line, not the
> _previous_ line.
>
>> @@ -98,23 +98,23 @@ struct async_cmd_info {
>>  struct nvme_queue {
>>       struct device *q_dmadev;
>>       struct nvme_dev *dev;
>> -     char irqname[24];       /* nvme4294967295-65535\0 */
>> -     spinlock_t q_lock;
>>       struct nvme_command *sq_cmds;
>> +     struct blk_mq_hw_ctx *hctx;
>>       volatile struct nvme_completion *cqes;
>> -     dma_addr_t sq_dma_addr;
>> -     dma_addr_t cq_dma_addr;
>>       u32 __iomem *q_db;
>>       u16 q_depth;
>> -     s16 cq_vector;
>>       u16 sq_head;
>>       u16 sq_tail;
>>       u16 cq_head;
>>       u16 qid;
>> +     s16 cq_vector;
>>       u8 cq_phase;
>>       u8 cqe_seen;
>> +     spinlock_t q_lock;
>>       struct async_cmd_info cmdinfo;
>> -     struct blk_mq_hw_ctx *hctx;
>> +     char irqname[24];       /* nvme4294967295-65535\0 */
>> +     dma_addr_t sq_dma_addr;
>> +     dma_addr_t cq_dma_addr;
>>  };
>>
>>  /*
>> --
>> 1.8.3.1
--
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