[systemd-devel] [PATCH 1/2] RFC: journald: Allow to cache the cg_get_root_path

[Karol Lewandowski]

On 07/16/2013 03:26 AM, Lennart Poettering wrote:
> On Mon, 08.07.13 18:39, Karol Lewandowski (k.lewandowsk at samsung.com) wrote:

>> According to my analysis /proc access is costly and it would
>> be best to cache the result for later use.  Difficulty comes
>> from trying to keep cache up to date, though.
> 
> We can't really cache this. This stuff is already racy, as by the time
> we read the attributes the process might already be gone.

Agreed, this is why I have been naively thinking that caching wouldn't
be that bad... However, sending all the required information in message
itself is clearly the best solution to this problem.

> I think the best way to deal with the performance issue here is the
> stuff discussed here:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=963620
> 
> i.e. just have the kernel augment our messages with the data we need,
> unconditionally. That way we fix both the race issue, and the
> performance issue, since the data is just there and we can make use of
> it without any further work.

Thanks a lot for this link!

Having this issue sorted out will allow me to take closer look
into "eliminating" 64-bit divisions in message receive path.
ARM doesn't support divide operation so every div[1] is being
done purely in software.

[1] Every div/% by non-constant value and every div/% by
    64-bit constant results in call to (slow) __aeabi_div*.

Thanks