linux-kernel - [PATCH 26/29] idr: Reduce the number of bits per level from 8 to 6

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Message-Id: <1479341856-30320-66-git-send-email-mawilcox@linuxonhyperv.com>
Date:   Wed, 16 Nov 2016 16:17:30 -0800
From:   Matthew Wilcox <mawilcox@...uxonhyperv.com>
To:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     linux-fsdevel@...r.kernel.org,
        Matthew Wilcox <willy@...ux.intel.com>, linux-mm@...ck.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCH 26/29] idr: Reduce the number of bits per level from 8 to 6

From: Matthew Wilcox <willy@...ux.intel.com>

In preparation for merging the IDR and radix tree, reduce the fanout at
each level from 256 to 64.  If this causes a performance problem then
a bisect will point to this commit, and we'll have a better idea about
what we might do to fix it.

Signed-off-by: Matthew Wilcox <willy@...ux.intel.com>
---
 include/linux/idr.h | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/include/linux/idr.h b/include/linux/idr.h
index 1eb755f..3c01b89 100644
--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -18,12 +18,11 @@
 #include <linux/rcupdate.h>
 
 /*
- * We want shallower trees and thus more bits covered at each layer.  8
- * bits gives us large enough first layer for most use cases and maximum
- * tree depth of 4.  Each idr_layer is slightly larger than 2k on 64bit and
- * 1k on 32bit.
+ * Using 6 bits at each layer allows us to allocate 7 layers out of each page.
+ * 8 bits only gave us 3 layers out of every pair of pages, which is less
+ * efficient except for trees with a largest element between 192-255 inclusive.
  */
-#define IDR_BITS 8
+#define IDR_BITS 6
 #define IDR_SIZE (1 << IDR_BITS)
 #define IDR_MASK ((1 << IDR_BITS)-1)
 
-- 
2.10.2