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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 11/27/19 6:05 AM, John Garry wrote:
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.

How can it be the same? The approach in the patchset needs to load
*tags->ptr, this one needs tags->ptr. That's the big difference.

--
Jens Axboe




[Index of Archives]     [Linux RAID]     [Linux SCSI]     [Linux ATA RAID]     [IDE]     [Linux Wireless]     [Linux Kernel]     [ATH6KL]     [Linux Bluetooth]     [Linux Netdev]     [Kernel Newbies]     [Security]     [Git]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Device Mapper]

  Powered by Linux