Message-ID: <4914DAEE.2050301@redhat.com>
Date: Fri, 07 Nov 2008 19:18:54 -0500
From: Masami Hiramatsu <mhiramat@redhat.com>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
       Jim Keniston <jkenisto@us.ibm.com>, David Miller <davem@davemloft.net>,
       LKML <linux-kernel@vger.kernel.org>, maneesh@in.ibm.com,
       Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH] kprobe: increase kprobe_hash_table size
References: <4914D2DE.9030603@redhat.com> <20081107155646.b375413e.akpm@linux-foundation.org>
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Hi Andrew,

Andrew Morton wrote:
> On Fri, 07 Nov 2008 18:44:30 -0500 Masami Hiramatsu <mhiramat@redhat.com> wrote:
> 
>> Increase the size of kprobe hash table to 512. It's useful when hundreds
>> of kprobes were used in the kernel because current size is just 64.
>>
> 
> "useful" is a bit vague.  How big is the problem which this solves, and
> how well did it solve it?

For example, when probing enters and exits of syscall-related functions,
we need more than 500 probes. In that case, each hlist would have 8
elements in average. With this patch, the hlist would have 1 element in
average.

I agree that there may be many opinions about what is the best suited size.
Why I chose 512 was that I thought the table (byte) size was less than or
equal 4096 even on 64-bit arch.

> See, someone (me) needs to decide whether to merge this and if so,
> whether to merge it into 2.6.29, 2.6.28, 2.6.27.x, 2.6.26.x and
> 2.6.25.x.  I'll need more information to make that decision, but I do
> not have it.

I think this improves performance just a bit.
So I think it would not be needed for 2.6.27.x or older kernel.

Thank you,

> 
>> --- 2.6.28-rc3.orig/kernel/kprobes.c
>> +++ 2.6.28-rc3/kernel/kprobes.c
>> @@ -49,7 +49,7 @@
>>  #include <asm/errno.h>
>>  #include <asm/uaccess.h>
>>
>> -#define KPROBE_HASH_BITS 6
>> +#define KPROBE_HASH_BITS 9
>>  #define KPROBE_TABLE_SIZE (1 << KPROBE_HASH_BITS)
> 
> 

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com

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