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
