Adaptive Re-sortable List (ARL)¶
This library allows netdata to read a series of
name - value pairs
in the fastest possible way.
ARLs are used all over netdata, as they are the most
CPU utilization efficient way to process
/proc files. They are used to
process both vertical (csv like) and horizontal (one pair per line)
name - value pairs.
How ARL works¶
It maintains a linked list of all
NAME (keywords), sorted in the
order found in the data source. The linked list is kept
sorted at all times - the data source may change at any time, the
linked list will adapt at the next iteration.
During initialization (just once), the caller:
arl_create()to create the ARL
arl_expect()multiple times to register the expected keywords
The library will call the
processor() function (given to
arl_create()), for each expected keyword found.
dst to be an
unsigned long long *.
name keyword may have a different
processor() (by calling
arl_expect_custom() instead of
Data collection iterations¶
For each iteration through the data source, the caller:
arl_begin()to initiate a data collection iteration. This is to be called just ONCE every time the source is re-evaluated.
arl_check()for each entry read from the file.
When the caller exits:
arl_free()to destroy this and free all memory.
ARL maintains a list of
name keywords found in the data source (even the ones
that are not useful for data collection).
If the data source maintains the same order on the
name-value pairs, for each
each call to
arl_check() only an
strcmp() is executed to verify the
expected order has not changed, a counter is incremented and a pointer is changed.
So, if the data source has 100
name-value pairs, and their order remains constant
over time, 100 successful
strcmp() are executed.
In the unlikely event that an iteration sees the data source with a different order, for each out-of-order keyword, a full search of the remaining keywords is made. But this search uses 32bit hashes, not string comparisons, so it should also be fast.
When all expectations are satisfied (even in the middle of an iteration),
the call to
arl_check() will return 1, to signal the caller to stop the loop,
saving valuable CPU resources for the rest of the data source.
In the following test we used alternative methods to process, 1M times,
a data source like
/proc/meminfo, already tokenized, in memory,
to extract the same number of expected metrics:
test|code|string comparison|number parsing|duration
strtoull()| 854.574 ms
Compared to unoptimized code (test No 1: 4.6sec):
- before ARL netdata was using test No 7 with hashing and a custom
str2ull()to achieve 602ms.
- the current ARL implementation is test No 9 that needs only 157ms (29 times faster vs unoptimized code, about 4 times faster vs optimized code).
Do not use ARL if the a name/keyword may appear more than once in the source data.