[RFC][PATCH]Small Minor optimization to kmem_cache_estimate

From: "BALBIR SINGH" <balbir.singh@wipro.com>
To: linux-kernel@vger.kernel.org
Subject: [RFC][PATCH]Small Minor optimization to kmem_cache_estimate
Date: Fri, 05 Oct 2001 17:04:21 +0530	[thread overview]
Message-ID: <3BBD9ABD.6040909@wipro.com> (raw)

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A small minor optimization in kmem_cache_estimate() slab.c

The patch below saves some CPU cycles, especially when the value of size
is small. This is against 2.4.2-2.

--- slab.c.org  Fri Oct  5 16:09:39 2001
+++ slab.c      Fri Oct  5 16:46:34 2001
@@ -386,10 +386,10 @@
                base = sizeof(slab_t);
                extra = sizeof(kmem_bufctl_t);
        }
-       i = 0;
+       i = (wastage - base)/(size + extra);
        while (i*size + L1_CACHE_ALIGN(base+i*extra) <= wastage)
                i++;
-       if (i > 0)
+       while (i*size + L1_CACHE_ALIGN(base+i*extra) > wastage)
                i--;

instead of looping through to get the right value, we make a guess
(mathematically) and move a bit to get the correct value.

Hey, I remember reading the Newton-Raphson method in school.

I verified the number of objects per slab is the same in both cases.
This patch may not improve the performance of your CPU by a great amount,
but when there is a faster way to do things, why live with the slower one.

NOTE: The code hardly does one loop in both the while statements.

Comments,
Balbir



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