On 2023/9/13 1:03, Eduard Zingerman wrote:
On Tue, 2023-09-12 at 09:40 -0700, Alexei Starovoitov wrote:
On Tue, Sep 12, 2023 at 7:19 AM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
On Tue, 2023-09-12 at 16:51 +0300, Eduard Zingerman wrote:
On Sat, 2023-09-09 at 02:16 -0700, Donglin Peng wrote:
Currently, we are only using the linear search method to find the type id
by the name, which has a time complexity of O(n). This change involves
sorting the names of btf types in ascending order and using binary search,
which has a time complexity of O(log(n)). This idea was inspired by the
following patch:
60443c88f3a8 ("kallsyms: Improve the performance of kallsyms_lookup_name()").
At present, this improvement is only for searching in vmlinux's and
module's BTFs, and the kind should only be BTF_KIND_FUNC or BTF_KIND_STRUCT.
Another change is the search direction, where we search the BTF first and
then its base, the type id of the first matched btf_type will be returned.
Here is a time-consuming result that finding all the type ids of 67,819 kernel
functions in vmlinux's BTF by their names:
Before: 17000 ms
After: 10 ms
The average lookup performance has improved about 1700x at the above scenario.
However, this change will consume more memory, for example, 67,819 kernel
functions will allocate about 530KB memory.
Hi Donglin,
I think this is a good improvement. However, I wonder, why did you
choose to have a separate name map for each BTF kind?
I did some analysis for my local testing kernel config and got such numbers:
- total number of BTF objects: 97350
- number of FUNC and STRUCT objects: 51597
- number of FUNC, STRUCT, UNION, ENUM, ENUM64, TYPEDEF, DATASEC objects: 56817
(these are all kinds for which lookup by name might make sense)
- number of named objects: 54246
- number of name collisions:
- unique names: 53985 counts
- 2 objects with the same name: 129 counts
- 3 objects with the same name: 3 counts
So, it appears that having a single map for all named objects makes
sense and would also simplify the implementation, what do you think?
Some more numbers for my config:
- 13241 types (struct, union, typedef, enum), log2 13241 = 13.7
- 43575 funcs, log2 43575 = 15.4
Thus, having separate map for types vs functions might save ~1.7
search iterations. Is this a significant slowdown in practice?
What do you propose to do in case of duplicates ?
func and struct can have the same name, but they will have two different
btf_ids. How do we store them ?
Also we might add global vars to BTF. Such request came up several times.
So we need to make sure our search approach scales to
func, struct, vars. I don't recall whether we search any other kinds.
Separate arrays for different kinds seems ok.
It's a bit of code complexity, but it's not an increase in memory.
Binary search gives, say, lowest index of a thing with name A, then
increment index while name remains A looking for correct kind.
Given the name conflicts info from above, 99% of times there would be
no need to iterate and in very few cases there would a couple of iterations.
Yeah, I agree.
Same logic would be necessary with current approach if different BTF
kinds would be allowed in BTF_ID_NAME_* cohorts. I figured that these
cohorts are mainly a way to split the tree for faster lookups, but
maybe that is not the main intent.
Yeah, I thought it may be faster for some kinds, but I didn't perform a
comparative test.
With 13k structs and 43k funcs it's 56k * (4 + 4) that's 0.5 Mbyte
extra memory. That's quite a bit. Anything we can do to compress it?
That's an interesting question, from the top of my head:
pre-sort in pahole (re-assign IDs so that increasing ID also would
mean "increasing" name), shouldn't be that difficult.
Thank you, I agree.
Folks requested vmlinux BTF to be a module, so it's loaded on demand.
BTF memory consumption is a concern to many.
I think before we add these per-kind search arrays we better make
BTF optional as a module.
