Dne 29.11.2016 v 09:16 Martin Wilck napsal(a):
On Tue, 2016-11-29 at 09:10 +0100, Zdenek Kabelac wrote:
Dne 29.11.2016 v 09:02 Martin Wilck napsal(a):
On Tue, 2016-11-29 at 07:47 +0100, Hannes Reinecke wrote:
On 11/28/2016 07:46 PM, Benjamin Marzinski wrote:
On Thu, Nov 24, 2016 at 10:21:10AM +0100, Martin Wilck wrote:
On Fri, 2016-11-18 at 16:26 -0600, Benjamin Marzinski wrote:
At any rate, I'd rather get rid of the gazillion waiter
threads
first.
Hm, I thought the threads are good because this avoids one
unresponsive
device to stall everything?
There is work making dm events pollable, so that you can wait
for
any
number of them with one thread. At the moment, once we get an
event, we
lock the vecs lock, which pretty much keeps everything else
from
running, so this doesn't really change that.
Which again leads me to the question:
Why are we waiting for dm events?
The code handling them is pretty arcane, and from what I've seen
there
is nothing in there which we wouldn't be informed via other
mechanisms
(path checker, uevents).
So why do we still bother with them?
I was asking myself the same question. From my inspection of the
kernel
code, there are two code paths that trigger a dm event but no
uevent
(bypass_pg() and switch_pg_num(), both related to path group
switching). If these are covered by the path checker, I see no
point in
waiting for DM events. But of course, I may be missing something.
Processing of 'dm' events likely should be postponed to 'dmeventd' -
which is a daemon resolving the problem here with waiting for an
event.
Plugin just takes the action.
IMHO there is nothing easier you can have.
It's then upto dmeventd to maintain the best 'connection' with kernel
and events.
But that would simply move the "gazillion waiter threads" from
multipathd to dmeventd, right? And it would introduce another boot
sequence dependency for multipathd, I'm not sure if that's desirable.
Well - frankly how many multipath devices have you seen on a single machine?
Have you noticed every single "XFS" mounted device spreads around 9 threads
these days?
But I'm not going to defend current 'thread' explosion with device monitoring
and this thing is BEING resolved.
The main point here is - multipathd does NOT need to solve this issue at all
and let dmeventd to resolve it - and once kernel will have better mechanism
with reliable event passing (which is NOT udev btw) - it will use it.
The solution is not that everyone will write everything here...
Regards
Zdenek
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