On Tue, 21 Aug 2012, Andreas Bluemle wrote:
> Hi Sage,
>
> as mentioned, the workload is a single sequential write on
> the client. The write is not O_DIRECT; and consequently
> the messages arrive at the OSD with 124 KByte per write request.
>
> The attached pdf shows a timing diagram of two concurrent
> write operations (one primary and one replication / secondary).
>
> The time spent on the primary write to get the OSD.:osd_lock
> releates nicely with the time when this lock is released by the
> secondary write.
Ah, I see.
There isn't a trivial way to pull osd_lock out of the picture; there are
several data structures it's protecting (pg_map, osdmaps, peer epoch map,
etc.). Before we try going down that road, I suspect it might be more
fruitful to see where cpu time is being spent while osd_lock is held.
How much of an issue does it look like this specific contention is for
you? Does it go away with larger writes?
sage
>
>
> Hope this helps
>
> Andreas
>
>
>
> Sage Weil wrote:
> > On Mon, 20 Aug 2012, Andreas Bluemle wrote:
> >
> > > Hi Sage,
> > >
> > > Sage Weil wrote:
> > >
> > > > Hi Andreas,
> > > >
> > > > On Thu, 16 Aug 2012, Andreas Bluemle wrote:
> > > >
> > > > > Hi,
> > > > >
> > > > > I have been trying to migrate a ceph cluster (ceph-0.48argonaut)
> > > > > to a high speed cluster network and encounter scalability problems:
> > > > > the overall performance of the ceph cluster does not scale well
> > > > > with an increase in the underlying networking speed.
> > > > >
> > > > > In short:
> > > > >
> > > > > I believe that the dispatching from SimpleMessenger to
> > > > > OSD worker queues causes that scalability issue.
> > > > >
> > > > > Question: is it possible that this dispatching is causing performance
> > > > > problems?
> > > > >
> > > > There is a single 'dispatch' thread that's processing this queue, and
> > > > conveniently perf lets you break down its profiling data on a per-thread
> > > > basis. Once you've ruled out the throttler as the culprit, you might
> > > > try
> > > > running the daemon with 'perf record -g -- ceph-osd ...' and then look
> > > > specifically at where that thread is spending its time. We shouldn't be
> > > > burning that much CPU just doing the sanity checks and then handing
> > > > requests
> > > > off to PGs...
> > > >
> > > > sage
> > > >
> > > >
> > > >
> > > The effect, which I am seeing, may be related to some locking issue.
> > > As I read the code, there are multiple dispatchers running: one per
> > > SimpleMessenger.
> > >
> > > On a typical OSD node, there is
> > >
> > > - the instance of the SimpleMessenger processing input from the client
> > > (primary writes)
> > > - other instances of SimpleMessenger, which process input from neighbor
> > > OSD
> > > nodes
> > >
> > > the latter generate replication writes to the OSD I am looking at.
> > >
> > > On the other hand, there is a single instance of the OSD object within the
> > > ceph-osd daemon.
> > > When dispatching messages to the OSD, then the OSD::osd_lock is held for
> > > the
> > > complete
> > > process of dispatching; see code below.
> > >
> > > When the write load increases, then multiple SimpleMessenger instances
> > > start
> > > to
> > > congest on the OSD::osd_lock.
> > > And this may cause delays in the individual dispatch process.
> > >
> >
> > This is definitely possible, yes, although it would surprise me if it's
> > happening here (unless your workload is all small writes). Just to confirm,
> > are you actually observing osd_lock contention, or speculating about what is
> > causing the delays you're seeing?
> >
> > I'm not sure what the best tool is to measure lock contention. Mark was
> > playing with a 'poor man's wall clock profiler' using stack trace sampling
> > from gdb. That would tell us whether threads were really blocking while
> > obtaining the osd_lock...
> >
> > Can you tell us a bit more about what your workload is?
> >
> > sage
> >
> >
> >
> > > bool OSD::ms_dispatch(Message *m)
> > > {
> > > // lock!
> > > osd_lock.Lock();
> > >
> > > while (dispatch_running) {
> > > dout(10) << "ms_dispatch waiting for other dispatch thread to complete" <<
> > > dendl;
> > > dispatch_cond.Wait(osd_lock);
> > > }
> > > dispatch_running = true;
> > >
> > > do_waiters();
> > > _dispatch(m);
> > > do_waiters();
> > >
> > > dispatch_running = false;
> > > dispatch_cond.Signal();
> > >
> > > osd_lock.Unlock();
> > > return true;
> > > }
> > >
> > >
> > >
> > > > > In detail:
> > > > >
> > > > > In order to find out more about this problem, I have added profiling
> > > > > to
> > > > > the ceph code in various place; for write operations to the primary or
> > > > > the
> > > > > secondary, timestamps are recorded for OSD object, offset and length
> > > > > of
> > > > > the such a write request.
> > > > >
> > > > > Timestamps record:
> > > > > - receipt time at SimpleMessenger
> > > > > - processing time at osd
> > > > > - for primary write operations: wait time until replication operation
> > > > > is acknowledged.
> > > > >
> > > > > What I believe is happening: dispatching requests from SimpleMessenger
> > > > > to
> > > > > OSD worker threads seems to consume a fair amount of time. This ends
> > > > > up in a widening gap between subsequent receipts of requests and the
> > > > > start
> > > > > of OSD processing them.
> > > > >
> > > > > A primary write suffers twice from this problem: first because
> > > > > the delay happens on the primary OSD and second because the
> > > > > replicating
> > > > > OSD also suffers from the same problem - and hence causes additional
> > > > > delays
> > > > > at the primary OSD when it waits for the commit from the replicating
> > > > > OSD.
> > > > >
> > > > > In the attached graphics, the x-axis shows the time (in seconds)
> > > > > The y-axis shows the offset where a request to write happened.
> > > > >
> > > > > The red bar represents the SimpleMessenger receive, i.e. from reading
> > > > > the message header until enqueuing the completely decoded message into
> > > > > the SImpleMessenger dispatch queue.
> > > > >
> > > > > The green bar represents the time required for local processing, i.e.
> > > > > dispatching the the OSD worker, writing to filesystem and journal,
> > > > > send
> > > > > out the replication operation to the replicating OSD. It right
> > > > > end of the green bar is the time when locally everything has finished
> > > > > and a commit could happen.
> > > > >
> > > > > The blue bar represents the time until the replicating OSD has sent a
> > > > > commit
> > > > > back to the primary OSD and the original write request can be
> > > > > committed to
> > > > > the client.
> > > > >
> > > > > The green bar is interrupted by a black bar: the left end represents
> > > > > the time when the request has been enqueued on the OSD worker queue.
> > > > > The
> > > > > right end gives the time when the request is taken off the OSD worker
> > > > > queue and actual OSD processing starts.
> > > > >
> > > > > The test was a simple sequential write to a rados block device.
> > > > >
> > > > > Receiption of the write requests at the OSD is also sequential in the
> > > > > graphics: the bar to the bottom of the graphics shows an earlier write
> > > > > request.
> > > > >
> > > > > Note that the dispatching of a later request in all cases relates to
> > > > > the
> > > > > enqueue time at the OSD worker queue of the previous write request:
> > > > > the
> > > > > left
> > > > > end of a black bar relates nicely to the beginning of a green bar
> > > > > above
> > > > > it.
> > > > >
> > > > >
> > > > >
> > > > > --
> > > > > Andreas Bluemle mailto:Andreas.Bluemle@xxxxxxxxxxx
> > > > > ITXperts GmbH http://www.itxperts.de
> > > > > Balanstrasse 73, Geb. 08 Phone: (+49) 89 89044917
> > > > > D-81541 Muenchen (Germany) Fax: (+49) 89 89044910
> > > > >
> > > > > Company details: http://www.itxperts.de/imprint.htm
> > > > >
> > > > >
> > > > >
> > > >
> > > --
> > > Andreas Bluemle mailto:Andreas.Bluemle@xxxxxxxxxxx
> > > ITXperts GmbH http://www.itxperts.de
> > > Balanstrasse 73, Geb. 08 Phone: (+49) 89 89044917
> > > D-81541 Muenchen (Germany) Fax: (+49) 89 89044910
> > >
> > > Company details: http://www.itxperts.de/imprint.htm
> > >
> > > --
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> > >
> > >
> >
> >
> >
> >
>
>
> --
> Andreas Bluemle mailto:Andreas.Bluemle@xxxxxxxxxxx
> ITXperts GmbH http://www.itxperts.de
> Balanstrasse 73, Geb. 08 Phone: (+49) 89 89044917
> D-81541 Muenchen (Germany) Fax: (+49) 89 89044910
>
> Company details: http://www.itxperts.de/imprint.htm
>
>
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