On 2022/10/29 16:10, Leizhen (ThunderTown) wrote:
>
>
> On 2022/10/27 14:27, Leizhen (ThunderTown) wrote:
>>
>>
>> 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.
>>>
>
> The following three hash algorithms are compared. The kernel is compiled by defconfig
> on arm64.
>
> |---------------------------------------------------------------------------------------|
> | | hash &= HASH_TABLE_SIZE - 1 |
> | | number of conflicts >= 1000 |
> |---------------------------------------------------------------------------------------|
> | ARRAY_SIZE(hash_table) | crc16 | jhash_one_at_a_time | string hash_32 |
> |---------------------------------------------------------------------------------------|
> | | 345b: 3905 | 0d40: 1013 | 4a4c: 6548 |
> | | 35bb: 1016 | 35ce: 6549 | 883a: 1015 |
> | 0x10000 | 385b: 6548 | 4440: 19126 | d05f: 19129 |
> | | f0ba: 19127 | 7ebe: 3916 | ecda: 3903 |
> |---------------------------------------------------------------------------------------|
> | | 0ba: 19168 | 440: 19165 | 05f: 19170 |
> | | 45b: 3955 | 5ce: 6577 | 83a: 1066 |
> | 0x1000 | 5bb: 1069 | d40: 1052 | a4c: 6609 |
> | | 85b: 6582 | ebe: 3938 | cda: 3924 |
> |---------------------------------------------------------------------------------------|
>
> Based on the above test results, I conclude that:
> 1. For the worst-case scenario, the three algorithms are not much different. But the kernel
> only implements crc16 and string hash_32. The latter is not processed byte-by-byte, so
> it is coupled with byte order and sizeof(long). So crc16 is the best choice.
> 2. For the worst-case scenario, there are almost 19K strings are mapped to the same hash
> value，just over 1/10 of the total. And with my current compression-then-comparison
> approach, it's 25-30 times faster. So there's still a need for my current approach, and
> they can be combined.
> if (nr_conflicts(key) >= CONST_N) {
> newname = compress(name);
> for_each_name_in_slot(key): compare(new_name)
> } else {
> for_each_name_in_slot(key): compare(name)
> }
>
> Above CONST_N can be roughly calculated:
> time_of_compress(name) + N * time_of_compare(new_name) <= N * time_of_compare(name)
> 3. For the worst-case scenario, there is no obvious difference between ARRAY_SIZE(hash_table)
> 0x10000 and 0x1000. So ARRAY_SIZE(hash_table)=0x1000 is enough.
> Statistic information:
> |------------------------------------------------------|
> | nr_conflicts(key) | ARRAY_SIZE(hash_table) |
> |------------------------------------------------------|
> | <= ? | 0x1000 | 0x10000 |
> |------------------------------------------------------|
> | 0 | 0 | 7821 |
> | 20 | 19 | 57375 |
> | 40 | 2419 | 124 |
> | 60 | 1343 | 70 |
> | 80 | 149 | 73 |
> | 100 | 38 | 49 |
> | 200 | 108 | 16 |
> | 400 | 14 | 2 |
> | 600 | 2 | 2 |
> | 800 | 0 | 0 |
> | 1000 | 0 | 0 |
> | 100000 | 4 | 4 |
> |------------------------------------------------------|
>
>
> Also, I re-calculated:
> Using hash will increase the memory size by up to "6 * kallsyms_num_syms + 4 * ARRAY_SIZE(hashtable)"
> |---- What I said earlier was 4
> The increased size is close to 1 MB if CONFIG_KALLSYMS_ALL=y.
>
> Hi, Luis:
> For the reasons of the above-mentioned second conclusion. And except for patches 4-6,
> even if only the hash method is used, other patches and option "--lto-clang" in patch 6/11
> are also needed. If you don't mind, I'd like to use hash at the next stage. The current
> patch set is already huge.
I just had an update in response to David Laight's email. The hash solution is like
a centrist. It doesn't seem very feasible.
Now, we need to make a decision. Choose one of the two:
1. Continue with my current approach. Improve the average performance of
kallsyms_lookup_name() by 20 to 30 times. The memory overhead is increased by:
arm64 (defconfig):
73.5KiB and 4.0% if CONFIG_KALLSYMS_ALL=y.
19.8KiB and 2.8% if CONFIG_KALLSYMS_ALL=n.
x86 (defconfig):
49.0KiB and 3.0% if CONFIG_KALLSYMS_ALL=y.
16.8KiB and 2.3% if CONFIG_KALLSYMS_ALL=n.
2. Sort names, binary search (The static function causes duplicate names. Additional work is required）
2^18=262144, only up to 18 symbol expansions and comparisons are required.
The performance is definitely excellent, although I haven't tested it yet.
The memory overhead is increased by: 6 * kallsyms_num_syms
arm64 (defconfig):
1MiB if CONFIG_KALLSYMS_ALL=y.
362KiB if CONFIG_KALLSYMS_ALL=n.
x86 (defconfig):
770KiB if CONFIG_KALLSYMS_ALL=y.
356KiB if CONFIG_KALLSYMS_ALL=n.
>
>
>>> 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
