Re: [PATCH v1 03/10] bcache: reduce cache_set devices iteration by devices_max_used

Michael Lyle
2018-01-03T17:11:50Z

On 01/03/2018 06:03 AM, Coly Li wrote:
> Member devices of struct cache_set is used to reference all attached
> bcache devices to this cache set. If it is treated as array of pointers,
> size of devices[] is indicated by member nr_uuids of struct cache_set.
>
> nr_uuids is calculated in drivers/md/super.c:bch_cache_set_alloc(),
> bucket_bytes(c) / sizeof(struct uuid_entry)
> Bucket size is determined by user space tool "make-bcache", by default it
> is 1024 sectors (defined in bcache-tools/make-bcache.c:main()). So default
> nr_uuids value is 4096 from the above calculation.
>
> Every time when bcache code iterates bcache devices of a cache set, all
> the 4096 pointers are checked even only 1 bcache device is attached to the
> cache set, that's a wast of time and unncessary.
>
> This patch adds a member devices_max_used to struct cache_set. Its value
> is 1 + the maximum used index of devices[] in a cache set. When iterating
> all valid bcache devices of a cache set, use c->devices_max_used in
> for-loop may reduce a lot of useless checking.
>
> Personally, my motivation of this patch is not for performance, I use it
> in bcache debugging, which helps me to narrow down the scape to check
> valid bcached devices of a cache set.

Oh, OK. I was going to argue that I didn't think this was worth it, but
I can see how it would be nice for inspecting memory and debug code.

>
> Signed-off-by: Coly Li <colyli@suse.de>

Reviewed-by: Michael Lyle <mlyle@lyle.org>