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

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Date:   Wed, 26 Apr 2017 15:08:18 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH v4 2/4] zram: implement deduplication in zram

On Wed, Apr 26, 2017 at 01:28:26PM +0900, Sergey Senozhatsky wrote:
> On (04/26/17 09:52), js1304@...il.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.