Re: [PATCH 1/5] MD: attach data to each bio

[NeilBrown <neilb@suse.com>]

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On Mon, Feb 13 2017, Christoph Hellwig wrote:

> On Fri, Feb 10, 2017 at 05:08:54PM +1100, NeilBrown wrote:
>> I must say that I don't really like this approach.
>> Temporarily modifying ->bi_private and ->bi_end_io seems
>> .... intrusive.   I suspect it works, but I wonder if it is really
>> robust in the long term.
>> 
>> How about a different approach..  Your main concern with my first patch
>> was that it called md_write_start() and md_write_end() much more often,
>> and these performed atomic ops on "global" variables, particular
>> writes_pending.
>> 
>> We could change writes_pending to a per-cpu array which we only count
>> occasionally when needed.  As writes_pending is updated often and
>> checked rarely, a per-cpu array which is summed on demand seems
>> appropriate.
>
> FYI, I much prefer you original approach, it's much closer to how
> the rest of the block stack works. 

I probably wasn't clear, but my intention was to stick with my original
approach, but make it more acceptable by removing the extra cost of
cache-line-bouncing that Shaohua correctly identified.
i.e. this patch was a preliminary to improve the original series.

Thanks,
NeilBrown

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