Re: Re: [PATCH -tip v7 24/26] kprobes: Enlarge hash table to 4096 entries

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2014/02/28 7:22), Masami Hiramatsu wrote:
> (2014/02/28 6:45), Andi Kleen wrote:
>> Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> writes:
>>
>>> Currently, since the kprobes expects to be used
>>> with less than 100 probe points, its hash table
>>> just has 64 entries. This is too little to handle
>>> several thousands of probes.
>>> Enlarge this to 4096 entires which just consumes
>>> 32KB (on 64bit arch) for better scalability.
>>
>> 32K for a debug feature that most systems never use seems
>> too large to me.
>>
>> First can you check if smaller hash tables work too
>> (perhaps with a better hash, like jhash) 
> 
> I doubt jhash helps it, but yes, at least the various size
> should be tested.

Here, I tested the hash table performance with 2^6 to 2^12.

Cycles% of get_kprobe with 10k probes:
Size	Cycles%
2^6	95.58%	
2^7	85.83%
2^8	68.43%
2^9	48.61%
2^10	46.95%
2^11	48.46%
2^12	56.95%

So, we can see the hash table larger than 2^9 (512 entries,
which consumes 4KB) has no performance improvement.
Would you think 4kB is still big for kprobes? :)

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com