Re: [PATCH v7 00/11] kallsyms: Optimizes the performance of lookup symbols

[Leizhen (ThunderTown)]

On 2022/10/27 11:26, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/10/27 3:03, Luis Chamberlain wrote:
>> On Wed, Oct 26, 2022 at 02:44:36PM +0800, Leizhen (ThunderTown) wrote:
>>> On 2022/10/26 1:53, Luis Chamberlain wrote:
>>>> This answers how we don't use a hash table, the question was *should* we
>>>> use one?
>>>
>>> I'm not the original author, and I can only answer now based on my understanding. Maybe
>>> the original author didn't think of the hash method, or he has weighed it out.
>>>
>>> Hash is a good solution if only performance is required and memory overhead is not
>>> considered. Using hash will increase the memory size by up to "4 * kallsyms_num_syms +
>>> 4 * ARRAY_SIZE(hashtable)" bytes, kallsyms_num_syms is about 1-2 million.

Sorry, 1-2 million ==> 0.1~0.2 million

>>>
>>> Because I don't know what hash algorithm will be used, the cost of generating the
>>> hash value corresponding to the symbol name is unknown now. But I think it's gonna
>>> be small. But it definitely needs a simpler algorithm, the tool needs to implement
>>> the same hash algorithm.
>>
>> For instance, you can look at evaluating if alloc_large_system_hash() would help.
> 
> OK, I found the right hash function. In this way, the tool does not need to consider
> the byte order.

https://en.wikipedia.org/wiki/Jenkins_hash_function

Let's go with jenkins_one_at_a_time_hash(), which looks simpler and doesn't even
have to think about sizeof(long). It seems to be closest to our current needs.

uint32_t jenkins_one_at_a_time_hash(const uint8_t* key, size_t length) {
	size_t i = 0;
	uint32_t hash = 0;

	while (i != length) {
		hash += key[i++];
		hash += hash << 10;
		hash ^= hash >> 6;
	}
	hash += hash << 3;
	hash ^= hash >> 11;
	hash += hash << 15;

	return hash;
}

> 
> include/linux/stringhash.h
> 
> /*
>  * Version 1: one byte at a time.  Example of use:
>  *
>  * unsigned long hash = init_name_hash;
>  * while (*p)
>  *      hash = partial_name_hash(tolower(*p++), hash);
>  * hash = end_name_hash(hash);
> 
> 
>>
>>   Luis
>> .
>>
> 

-- 
Regards,
  Zhen Lei