linux-kernel - Re: [PATCH] mm: extend zero pages to same element pages for zram

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Date:   Tue, 24 Jan 2017 15:58:02 +0800
From:   zhouxianrong <zhouxianrong@...wei.com>
To:     Minchan Kim <minchan@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
CC:     Joonsoo Kim <iamjoonsoo.kim@....com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <sergey.senozhatsky@...il.com>, <ngupta@...are.org>,
        <Mi.Sophia.Wang@...wei.com>, <zhouxiyu@...wei.com>,
        <weidu.du@...wei.com>, <zhangshiming5@...wei.com>,
        <won.ho.park@...wei.com>
Subject: Re: [PATCH] mm: extend zero pages to same element pages for zram

@@ -161,15 +161,55 @@ static bool page_zero_filled(void *ptr)
  {
  	unsigned int pos;
  	unsigned long *page;
+	static unsigned long total;
+	static unsigned long zero;
+	static unsigned long pattern_char;
+	static unsigned long pattern_short;
+	static unsigned long pattern_int;
+	static unsigned long pattern_long;
+	unsigned char *p_char;
+	unsigned short *p_short;
+	unsigned int *p_int;
+	bool retval = false;
+
+	++total;

  	page = (unsigned long *)ptr;

-	for (pos = 0; pos != PAGE_SIZE / sizeof(*page); pos++) {
-		if (page[pos])
-			return false;
+	for (pos = 0; pos < PAGE_SIZE / sizeof(unsigned long) - 1; ++pos) {
+	       if (page[pos] != page[pos + 1])
+	                return false;
  	}

-	return true;
+	p_char = (unsigned char *)ptr;
+	p_short = (unsigned short *)ptr;
+	p_int = (unsigned int *)ptr;
+
+	if (page[0] == 0) {
+		++zero;
+		retval = true;
+	} else if (p_char[0] == p_char[1] &&
+		       p_char[1] == p_char[2] &&
+		       p_char[2] == p_char[3] &&
+		       p_char[3] == p_char[4] &&
+		       p_char[4] == p_char[5] &&
+		       p_char[5] == p_char[6] &&
+		       p_char[6] == p_char[7])
+		++pattern_char;
+	else if (p_short[0] == p_short[1] &&
+		       p_short[1] == p_short[2] &&
+		       p_short[2] == p_short[3])
+		++pattern_short;
+	else if (p_int[0] == p_int[1] &&
+		       p_int[1] == p_int[2])
+		++pattern_int;
+	else {
+		++pattern_long;
+	}
+
+	pr_err("%lld %lld %lld %lld %lld %lld\n", zero, pattern_char, pattern_short, pattern_int, pattern_long, total);
+
+	return retval;
  }

the result as listed below:

zero    pattern_char   pattern_short   pattern_int   pattern_long   total      (unit)
162989  14454          3534            23516         2769           3294399    (page)

statistics for the result:

          pattern zero  pattern char  pattern short  pattern int  pattern long
AVERAGE  0.745696298   0.085937175   0.015957701    0.131874915  0.020533911
STDEV    0.035623777   0.016892402   0.004454534    0.021657123  0.019420072
MAX      0.973813421   0.222222222   0.021409518    0.211812245  0.176512625
MIN      0.645431905   0.004634398   0              0            0


On 2017/1/23 15:40, Minchan Kim wrote:
> On Mon, Jan 23, 2017 at 04:13:39PM +0900, Sergey Senozhatsky wrote:
>> On (01/23/17 15:27), Joonsoo Kim wrote:
>>> Hello,
>>>
>>> Think about following case in 64 bits kernel.
>>>
>>> If value pattern in the page is like as following, we cannot detect
>>> the same page with 'unsigned int' element.
>>>
>>> AAAAAAAABBBBBBBBAAAAAAAABBBBBBBB...
>>>
>>> 4 bytes is 0xAAAAAAAA and next 4 bytes is 0xBBBBBBBB and so on.
>>
>> yep, that's exactly the case that I though would be broken
>> with a 4-bytes pattern matching. so my conlusion was that
>> for 4 byte pattern we would have working detection anyway,
>> for 8 bytes patterns we might have some extra matching.
>> not sure if it matters that much though.
>
> It would be better for deduplication as pattern coverage is bigger
> and we cannot guess all of patterns now so it would be never ending
> story(i.e., someone claims 16bytes pattern matching would be better).
> So, I want to make that path fast rather than increasing dedup ratio
> if memset is really fast rather than open-looping. So in future,
> if we can prove bigger pattern can increase dedup ratio a lot, then,
> we could consider to extend it at the cost of make that path slow.
>
> In summary, zhouxianrong, please test pattern as Joonsoo asked.
> So if there are not much benefit with 'long', let's go to the
> 'int' with memset. And Please resend patch if anyone dosn't oppose
> strongly by the time.
>
> Thanks.
>
>
> .
>