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

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Date:   Wed, 26 Apr 2017 13:28:26 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     js1304@...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,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v4 2/4] zram: implement deduplication in zram

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.

	-ss