Re: [PATCH 3/8] blk-mq: Use a pointer for sbitmap

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On 27/11/2019 01:46, Jens Axboe wrote:
Would be interesting to check the generated code for that, ideally we'd
get rid of the extra load for that case, even if it is in the same
cacheline.

I checked the disassembly and we still have the load instead of the add.

This is not surprising, as the compiler would not know for certain that
we point to a field within the same struct. But at least we still should
point to a close memory.

Note that the pointer could be dropped, which would remove the load, but
then we have many if-elses which could be slower, not to mention that
the blk-mq-tag code deals in bitmap pointers anyway.

Hi Jens,

It might still be worthwhile to do:

if (tags->ptr == &tags->__default)
	foo(&tags->__default);

to make it clear, as that branch will predict easily.

Not sure. So this code does produce the same assembly, as we still need to do the tags->ptr load for the comparison.

And then if you consider blk_mq_get_tags() as an example, there is no other hot value available to indicate whether the shared tags are used to decide whether to use &tags->__default.

I'll consider it more.

If if can be done
in a nice enough fashion and not sprinkled everywhere, in some fashion.

Should be testable, though.

-- Jens Axboe

Thanks,
John



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