D-Bus core due for a release?

Tim Dijkstra newsuser at famdijkstra.org
Thu May 17 04:51:49 PDT 2007

On Wed, 16 May 2007 18:46:33 -0400
Havoc Pennington <hp at redhat.com> wrote:

> Hi,
> Tim Dijkstra wrote:
> > On Wed, 16 May 2007 14:41:29 -0400
> > Havoc Pennington <hp at redhat.com> wrote:
> > 
> >>> * possibly the user database (it's unclear whether this made it into 1.0.2)  
> >> Not sure I remember the current state here, but I thought there was a 
> >> patch that needed some performance testing before we put it in.
> > 
> > Part of it is cleanup; there used to be two copies of the cache, also
> > they didn't got expired on dbus reload. The second part is a configure
> > switch to disable it altogether, which is the correct thing to do IMHO.
> > It will indeed hit performance if you use for example an LDAP server
> > _without_ nscd. If you install nscd I couldn't measure a difference. I
> > think we shouldn't redesign caching if there is already a caching
> > daemom.
> > 
> Thanks, according to the changelog this patch is in HEAD already (did 
> not check 1.0.x). So the issue is just the default for the configure 
> switch (or I guess if we change to default to no cache, arguably we 
> should just nuke all this code).
> I don't think it's only LDAP - it's far too slow with /etc/passwd also I 
> think, without nscd.

IIRC in the tests I did with just plain /etc/passwd, I couldn't find
any significant slowdown. Note that this test was not very
sophisticated, something like `make test' or so... Anyway it would be
good if somebody else then me did some tests with it.

> How does the nscd caching work I wonder - remember on *every* message we 
> can do things like get the groups a user is in. So we need that info 
> in-process. Does libc keep a client-side/in-process cache also when 
> using nscd?

There is an option called `shared' (configurable in nscd.conf) which is
standard on. Found some info [0] which I didn't verify myself:

 If sharing is disabled for a name service, nscd clients use their 
 connection to /var/run/nscd/socket for all access to the name service cache.

 If sharing is enabled for a name service, an nscd client will retrieve a 
 file descriptor for nscd's corresponding mmap'd cache file over the 
 /var/run/nscd/socket, and will then mmap that file itself.  After that, 
 any lookups for entries which are present in the cache and have a good 
 time-to-live can be satisfied by the client without (talking to/waiting 
 for) nscd.  Lookups on entries which are either not present in the cache 
 or have an expired TTL still require interactions with nscd.

> Is there some way to ask libc "is nscd being used"?

I don't know. Will have to look that one up...

grts Tim

[0] http://anusf.anu.edu.au/~djh900/nscd.html
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