[ofa-general] Re: Re: openSM: Different IB MTUs
Michael S. Tsirkin
mst at dev.mellanox.co.il
Thu Jul 26 00:22:45 PDT 2007
What does "1" mean? Surely not 1 byte MTU :)
IMO a good format would be the MTU value in bytes.
E.g. 512, 1024, 2048, 4096.
Quoting Shirley Ma <xma at us.ibm.com>:
Subject: RE: Re: openSM: Different IB MTUs
Eitan,
That's a good approach to address the issue.
thanks
Shirley Ma
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"Eitan Zahavi" [cid] *
<eitan at mellanox.co.il> To Shirley Ma/Beaverton/IBM at IBMUS
[cid] *
07/25/07 11:00 PM cc <general at lists.openfabrics.org>, "Hal Rosenstock"
<hal.rosenstock at gmail.com>
[cid] *
Subject RE: [ofa-general] Re: openSM: Different IB MTUs
* *
I propose that when there is no MTU in the partition policy file OpenSM use a
configurable default from: /etc/cache/opensm/opensm.opt.
Something like:
# The default MTU to be used for IPoIB and other MCGs when the partition-policy
# does not provide exact value. The default is the lowest possible MTU
mcg_default_mtu 1
Eitan Zahavi
Senior Engineering Director, Software Architect
Mellanox Technologies LTD
Tel:+972-4-9097208
Fax:+972-4-9593245
P.O. Box 586 Yokneam 20692 ISRAEL
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From: Shirley Ma [mailto:xma at us.ibm.com]
Sent: Wednesday, July 25, 2007 10:45 PM
To: Eitan Zahavi
Cc: general at lists.openfabrics.org; Hal Rosenstock
Subject: RE: [ofa-general] Re: openSM: Different IB MTUs
Hello Eitan, Hal,
Thanks. It's good openSM has the configuration option to set up these
attributes in MC. Is this a good idea to add below to openSM: When there is no
MTU defined in the configuration file, SM can pick up the smallest link MTU in
the fabrics by default? MTU is unlikely rate, slower rate might indicate the
cablling problem. So using the smallest link MTU in the fabrics might not be a
bad choice for MC by default. The reason I request here is to create IP
multicast group, MTU is not an attribute of the group. When mapping IP
multicast to IB multicast, IB muliticast might fail because of different IB
link MTU size in the group, but IP multicast group will be successful without
knowing the failure. If admin sets MTU in configuration file, admin would know
this failure. Otherwise, admin/users could spend too much time on debugging
their broken multicasting applications.
Thanks
Shirley Ma
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<eitan at mellanox.co.il> To "Hal Rosenstock"
<hal.rosenstock at gmail.com>, Shirley
07/25/07 12:25 PM Ma/Beaverton/IBM at IBMUS
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cc <general at lists.openfabrics.org>
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Subject RE: [ofa-general] Re: openSM:
Different IB MTUs
* *
Hi Shirley,
I think I understand where your question comes from...
Many have issue with heterogonous fabrics where not all nodes have same MTU or
Speed.
Especially when IPoIB relies on all nodes joining the broadcast group.
The term "join" for multicast groups is a little overloaded.
If a node joins an existing MC group it has to have a rate (speed * width) >
MCG.rate and support MTU > MCG.MTU otherwise it is denied.
If the join is actually a "create" the node has to provide the rate and MTU
which define the MCG values.
To allow for administrator to control the IPoIB MCGs MTU and rate OpenSM
provides the means to control these
values per partition. See the doc/partition-config.doc
Still the administrator should know what would be the lowest MTU and rate the
nodes expected to join the IPoIB subnet have.
The tradeoff is in the hands of the administrator who can set a value that will
prevent slow nodes from joining the group,
or assign a low value that will fit all nodes but slow down communication ...
EZ
Eitan Zahavi
Senior Engineering Director, Software Architect
Mellanox Technologies LTD
Tel:+972-4-9097208
Fax:+972-4-9593245
P.O. Box 586 Yokneam 20692 ISRAEL
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
From: general-bounces at lists.openfabrics.org [
mailto:general-bounces at lists.openfabrics.org] On Behalf Of Hal Rosenstock
Sent: Wednesday, July 25, 2007 10:01 PM
To: Shirley Ma
Cc: general at lists.openfabrics.org
Subject: [ofa-general] Re: openSM: Different IB MTUs
Shirley,
On 7/25/07, Shirley Ma <xma at us.ibm.com> wrote:
Hal,
Thanks for your prompt reply. I am asking for how openSM handle
different link MTUs in SA MCMemberRecord MTU. For example, if we have
some links MTU as 2K, some links MTU as 1K. Then when enabling IPoIB,
how does SM decide IPoIB broadcast group MCMemberRecord MTU size? When
creating an IB multicast group from a 2K MTU node first, which PMTU
value is attaching to this IB multicast group MCMemberRecord MTU?
MCMemberRecord MTU gets the group MTU (when created). This is either this first
joiner with sufficient components or preconfigured (and MTU can be set in the
config). If a joiner has insufficient MTU for the group, it is denied.
-- Hal
Thanks
Shirley Ma
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hal.rosenstock at gmail.com> To Shirley Ma/Beaverton/
IBM at IBMUS
07/25/07 10:57 AM [cid] *
cc general at lists.openfabrics.org
[cid] *
Subject Re: openSM: Different IB MTUs
* *
Shirley,
On 7/25/07, Shirley Ma < xma at us.ibm.com> wrote:
Hello Hal,
How does openSM handle CAs with different MTUs in the
same subnet? For example, IPoIB broadcast group MTU, IB
multicast group PMTU? Does openSM pick up the smallest
MTU in the subnet?
Are you asking about link MTU, SA PathRecord/MultiPathRecord MTU, SA
MCMemberRecord MTU, or all of these ?
-- Hal
Thanks
Shirley Ma
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