lkml.org 
[lkml]   [2006]   [Jan]   [29]   [last100]   RSS Feed
Views: [more markup]  [less markup]  [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: RCU latency regression in 2.6.16-rc1
FromLee Revell <>
DateSun, 29 Jan 2006 02:38:02 -0500
On Sat, 2006-01-28 at 20:52 +0100, Eric Dumazet wrote:
> > Your new trace shows that we are held up in in rt_run_flush(). 
> > I guess we need to investigate why we spend so much time in rt_run_flush(),
> > because of a big route table or the lock acquisitions.> > Some machines have millions of entries in their route cache.
> > I suspect we cannot queue all them (or only hash heads as your
> previous patch) by RCU. Latencies and/or OOM can occur.
> > What can be done is :> > in rt_run_flush(), allocate a new empty hash table, and exchange the
> hash tables.> > Then wait a quiescent/grace RCU period (may be the exact term is not
> this one, sorry, I'm not RCU expert)> > Then free all the entries from the old hash table (direclty of course,
> no need for RCU grace period), and free the hash table.
> > As the hash table can be huge, we might need allocate it at boot time,
> just in case a flush is needed (it usually is :) ). If we choose
> dynamic allocation and this allocation fails, then fallback to what is
> done today.> 

No problem, I'm not a networking expert...

Ingo's response to these traces was that softirq preemption, which
simply offloads all softirq processing to softirqd and has been tested
in the -rt patchset for over a year, is the easiest solution.  Any
thoughts on that?  Personally, I'd rather fix the very few problematic
softirqs, than take such a drastic step - this softirq appears to be one
of the last obstacles to being able to meet a 1ms soft RT constraint
with the mainline kernel.

Thanks for looking at this; I'd be glad to test any patches...

Lee


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-01-29 08:45    [from the cache]
©2003-2008