On Wed, 23 Jul 2025 10:46:12 -0700
Thorsten Blum <thorsten.b...@linux.dev> wrote:
> Your commit fca8300f68fe3 changed it from __dynamic_array() to __array()
> and __string() seems to be just a special version of __dynamic_array()
> with a length of -1.
>
> In the commit description you wrote: "Since the size of the name is at
> most 16 bytes (defined by IFNAMSIZ), it is not worth spending the effort
> to determine the size of the string."
So the original had:
__dynamic_array(char, name, IFNAMSIZ )
Which is not dynamic at all. A dynamic_array (like __string) saves the
size in meta data within the event. So basically the above is wasting
bytes to save a fixed size. If you are going to use a dynamic array,
might as well make it dynamic!
I was doing various clean ups back then so I didn't look too deeply
into this event when I made that change. I just saw the obvious waste
of space in the ring buffer.
Just to explain it in more detail. A dynamic_array has in the ring buffer:
short offset;
short len;
[..]
char name[len];
That is, 4 bytes are used to know the size of the array and where in
the event it is located. Thus the __dynamic_array() usage basically had:
short offset;
short len = IFNAMSIZ;
[..]
char name[IFNAMSIZ];
Why have the offset and length? with just __array(char, name, IFNAMSIZ}
it would be just:
char name[IFNAMSIZ];
See why I changed it?
Now, the change I'm suggesting now would make the __string() be dynamic!
short offset;
short len = strlen(res->nh && res->nh->fib_nh_dev ?
res->nh->fib_nh_dev->name : "-") + 1;
[..]
char name[len];
As IFNAMSIZ is 16, and the above adds 4 bytes to the name, if the name
is less than 7 bytes or less, you save memory on the ring buffer.
2 bytes: offset
2 bytes: len;
7 bytes + '\0'
total: 12 bytes
Note, if there's only one dynamic value, it is always at least 4 bytes aligned.
-- Steve
