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

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Date:   Wed, 26 Apr 2017 15:14:57 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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 (04/26/17 15:08), Joonsoo Kim 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.

oh, my bad. you are right. somehow I didn't spot the unlock.

	-ss