From: Nitin Gupta <ngupta@vflare.org>
To: Greg KH <greg@kroah.com>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
Seth Jennings <sjenning@linux.vnet.ibm.com>,
Minchan Kim <minchan.kim@gmail.com>,
linux-mm <linux-mm@kvack.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: [PATCH] zsmalloc documentation
Date: Sun, 3 Jun 2012 13:20:05 -0700 [thread overview]
Message-ID: <1338754805-1733-1-git-send-email-ngupta@vflare.org> (raw)
Signed-off-by: Nitin Gupta <ngupta@vflare.org>
---
drivers/staging/zsmalloc/zsmalloc-main.c | 49 ++++++++++++++++++++++++++++++
1 file changed, 49 insertions(+)
diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
index 4496737..f80f2fd 100644
--- a/drivers/staging/zsmalloc/zsmalloc-main.c
+++ b/drivers/staging/zsmalloc/zsmalloc-main.c
@@ -10,6 +10,55 @@
* Released under the terms of GNU General Public License Version 2.0
*/
+
+/*
+ * This allocator is designed for use with zcache and zram. Thus, the
+ * allocator is supposed to work well under low memory conditions. In
+ * particular, it never attempts higher order page allocation which is
+ * very likely to fail under memory pressure. On the other hand, if we
+ * just use single (0-order) pages, it would suffer from very high
+ * fragmentation -- any object of size PAGE_SIZE/2 or larger would occupy
+ * an entire page. This was one of the major issues with its predecessor
+ * (xvmalloc).
+ *
+ * To overcome these issues, zsmalloc allocates a bunch of 0-order pages
+ * and links them together using various 'struct page' fields. These linked
+ * pages act as a single higher-order page i.e. an object can span 0-order
+ * page boundaries. The code refers to these linked pages as a single entity
+ * called zspage.
+ *
+ * Following is how we use various fields and flags of underlying
+ * struct page(s) to form a zspage.
+ *
+ * Usage of struct page fields:
+ * page->first_page: points to the first component (0-order) page
+ * [first] page->private (union with page->first_page): refers to the
+ * component page after the first page
+ *
+ * [first] page->freelist: points to the first free object in zspage.
+ * Free objects are linked together using in-place
+ * metadata.
+ * page->index (union with page->freelist): offset of the first object
+ * starting in this page. For the first page, this is
+ * always 0, so we use this field (aka freelist) to point
+ * to the first free object in zspage.
+ *
+ * [first] page->objects: maximum number of objects we can store in this
+ * zspage (class->zspage_order * PAGE_SIZE / class->size)
+ *
+ * page->lru: links together all component pages (except the first page)
+ * of a zspage
+ * [first] page->lru: links together first pages of various zspages.
+ * Basically forming list of zspages in a fullness group.
+ *
+ * [first] page->mapping: class index and fullness group of the zspage
+ *
+ * Usage of struct page flags:
+ * PG_private: identifies the first component page
+ * PG_private2: identifies the last component page
+ *
+ */
+
#ifdef CONFIG_ZSMALLOC_DEBUG
#define DEBUG
#endif
--
1.7.10.2
next reply other threads:[~2012-06-08 17:50 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-06-03 20:20 Nitin Gupta [this message]
2012-06-09 11:29 ` [PATCH] zsmalloc documentation Konrad Rzeszutek Wilk
2012-06-10 0:04 ` Nitin Gupta
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