[systemd-devel] [PATCH v5] journalctl: Add support for showing messages from a previous boot

[Lennart Poettering]

On Fri, 28.06.13 17:26, Jan Janssen (medhefgo at web.de) wrote:

> Unfortunately, order of sd_journal_enumerate_unique() is
> undefined, so we have to sort the boot IDs. For that we
> seach for any log entry with a specific boot ID and then
> use the realtime stamp to order everything.
> ---
> 
> Hi,
> 
> I redid the boot ID look up to use enumerate_unique.
> 
> This is quite fast if the cache is warm but painfully slow if
> it isn't. It has a slight chance of returning the wrong order if
> realtime clock jumps around.
> 
> This one has to do n searches for every boot ID there is plus
> a sort, so it depends heavily on cache hotness. This is in contrast
> to the other way of look-up through filtering by a MESSAGE_ID,
> which only needs about 1 seek + whatever amount of relative IDs
> you want to walk.
> 
> I also have a linked-list + (in-place) mergesort version of this
> patch, which has pretty much the same runtime. But since this one
> is using libc sorting and armortized allocation, I prefer this
> one.
> 
> To summarize: The MESSAGE_ID way is a *lot* faster but can be
> incomplete due to rotation, while the enumerate+sort will find
> every boot ID out there but will be painfully slow for large
> journals and cold caches.
> 
> You choose :P

Applied this one now. If people start complaining about its speed we can
reinvestigate and do find some way for optimization...

Thanks,

Lennart

-- 
Lennart Poettering - Red Hat, Inc.