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Subject: Re: [OMPI devel] Device failover on ob1
From: Ralph Castain (rhc_at_[hidden])
Date: 2009-08-02 18:19:56


The objections being cited are somewhat unfair - perhaps people do not
understand the proposal being made? The developers have gone out of
their way to ensure that all changes are configured out unless you
specifically select to use that functionality. This has been our
policy from day one - as long as the changes have zero impact unless
the user specifically requests that it be used, then no harm is done.

So I personally don't see any objection to bringing it into the code
base. Latency is not impacted one bit -unless- someone deliberately
configures the code to use this feature. In that case, they are
deliberately accepting any impact in order to gain the benefits.

Perhaps a bigger question needs to be addressed - namely, does the ob1
code need to be refactored?

Having been involved a little in the early discussion with bull when
we debated over where to put this, I know the primary concern was that
the code not suffer the same fate as the dr module. We have since run
into a similar issue with the checksum module, so I know where they
are coming from.

The problem is that the code base is adjusted to support changes in
ob1, which is still being debugged. On the order of 95% of the code in
ob1 is required to be common across all the pml modules, so the rest
of us have to (a) watch carefully all the commits to see if someone
touches ob1, and then (b) manually mirror the change in our modules.

This is not a supportable model over the long-term, which is why dr
has died, and checksum is considering integrating into ob1 using
configure #if's to avoid impacting non-checksum users. Likewise,
device failover has been treated similarly here - i.e., configure out
the added code unless someone wants it.

This -does- lead to messier source code with these #if's in it. If we
can refactor the ob1 code so the common functionality resides in the
base, then perhaps we can avoid this problem.

Is it possible?
Ralph

On Aug 2, 2009, at 3:25 PM, Graham, Richard L. wrote:

>
>
>
> On 8/2/09 12:55 AM, "Brian Barrett" <brbarret_at_[hidden]> wrote:
>
> While I agree that performance impact (latency in this case) is
> important, I disagree that this necessarily belongs somewhere other
> than ob1. For example, a zero-performance impact solution would be to
> provide two versions of all the interface functions, one with failover
> turned on and one with it turned off, and select the appropriate
> functions at initialization time. There are others, including careful
> placement of decision logic, which are likely to result in near-zero
> impact. I'm not attempting to prescribe a solution, but refuting the
> claim that this can't be in ob1 - I think more data is needed before
> such a claim is made.
>
>>> Just another way to do handle set the function pointers.
>
> Mouhamed - can the openib btl try to re-establish a connection between
> two peers today (with your ob1 patches, obviously)? Would this allow
> us to adapt to changing routes due to switch failures (assuming that
> there are other physical routes around the failed switch, of course)?
>
>>> The big question is what are the assumptions that are being made
>>> for this mode of failure recovery. If the assumption is that
>>> local completion
>>> implies remote delivery, the problem is simple to solve. If not,
>>> heavier
>>> weight protocols need to be used to cover the range of ways failure
>>> may manifest itself.
>
> Rich
>
> Thanks,
>
> Brian
>
> On Aug 1, 2009, at 6:21 PM, Graham, Richard L. wrote:
>
>> What is the impact on sm, which is by far the most sensitive to
>> latency. This really belongs in a place other than ob1. Ob1 is
>> supposed to provide the lowest latency possible, and other pml's are
>> supposed to be used for heavier weight protocols.
>>
>> On the technical side, how do you distinguish between a lot
>> acknowledgement and an undelivered message ? You really don't want
>> to try and deliver data into user space twice, as once a receive is
>> complete, who knows what the user has done with that buffer ? A
>> general treatment needs to be able to false negatives, and attempts
>> to deliver the data more than once.
>>
>> How are you detecting missing acknowledgements ? Are you using some
>> sort of timer ?
>>
>> Rich
>>
>> On 7/31/09 5:49 AM, "Mouhamed Gueye" <mouhamed.gueye_at_[hidden]> wrote:
>>
>> Hi list,
>>
>> Here is an update on our work concerning device failover.
>>
>> As many of you suggested, we reoriented our work on ob1 rather than
>> dr
>> and we now have a working prototype on top of ob1. The approach is to
>> store btl descriptors sent to peers and delete them when we receive
>> proof of delivery. So far, we rely on completion callback functions,
>> assuming that the message is delivered when the completion function
>> is
>> called, that is the case of openib. When a btl module fails, it is
>> removed from the endpoint's btl list and the next one is used to
>> retransmit stored descriptors. No extra-message is transmitted, it
>> only
>> consists in additions to the header. It has been mainly tested with
>> two
>> IB modules, in both multi-rail (two separate networks) and multi-
>> path (a
>> big unique network).
>>
>> You can grab and test the patch here (applies on top of the trunk) :
>> http://bitbucket.org/gueyem/ob1-failover/
>>
>> To compile with failover support, just define --enable-device-
>> failover
>> at configure. You can then run a benchmark, disconnect a port and see
>> the failover operate.
>>
>> A little latency increase (~ 2%) is induced by the failover layer
>> when
>> no failover occurs. To accelerate the failover process on openib, you
>> can try to lower the btl_openib_ib_timeout openib parameter to 15 for
>> example instead of 20 (default value).
>>
>> Mouhamed
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>>
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>
> --
> Brian Barrett
> Open MPI developer
> http://www.open-mpi.org/
>
>
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