[systemd-devel] Hashmap iteration is too costly

[Lennart Poettering]

On Fri, 14.07.17 03:18, vcaputo at pengaru.com (vcaputo at pengaru.com) wrote:

> The current hashmap iteration incurs at least one function call per
> iteration, and in my observations using gcc 6.3 w/-g -O2, it's two:
> 
>  internal_hashmap_iterate()
>  hashmap_iterate_entry()
> 
> for every element in the hashmap.
> 
> In profiles of `journalctl -b --no-pager` having multiple (50) log files,
> the hashmap iteration shows up as ~10% of the CPU in callgrind
> output.

I wonder if it wouldn't be easiest to simply bypass hashmap native
iteration for this case. i.e. add a linked list for the open journal
files which we can easily traverse in O(1) time?

The hashmap implementation has been optimized to require very little
memory, because we have so many hashmap/sets around in PID 1. It's not
optimized to make iteration fast though.

Lennart

-- 
Lennart Poettering, Red Hat