Re: [PATCH v4 2/4] zram: implement deduplication in zram

[Joonsoo Kim <js1304@gmail.com>]

On Wed, Apr 26, 2017 at 01:28:26PM +0900, Sergey Senozhatsky wrote:
> On (04/26/17 09:52), js1304@gmail.com wrote:
> [..]
> > +struct zram_hash {
> > +	spinlock_t lock;
> > +	struct rb_root rb_root;
> >  };
> 
> just a note.
> 
> we can easily have N CPUs spinning on ->lock for __zram_dedup_get() lookup,
> which can invole a potentially slow zcomp_decompress() [zlib, for example,
> with 64k pages] and memcmp(). the larger PAGE_SHIFT is, the more serialized
> IOs become. in theory, at least.
> 
> CPU0				CPU1		...	CPUN
> 
> __zram_bvec_write()	__zram_bvec_write()		__zram_bvec_write()
>  zram_dedup_find()	 zram_dedup_find()		 zram_dedup_find()
>   spin_lock(&hash->lock);
> 			  spin_lock(&hash->lock);	  spin_lock(&hash->lock);
>    __zram_dedup_get()
>     zcomp_decompress()
>      ...
> 
> 
> so may be there is a way to use read-write lock instead on spinlock for hash
> and reduce write/read IO serialization.

In fact, dedup release hash->lock before doing zcomp_decompress(). So,
above contention cannot happen.

However, contention still possible when traversing the rb_tree. If
your fio shows that contention, I will change it to read-write lock.

Thanks.