From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Jim Schutt" Subject: [PATCH v4 11/18] opensm: Update documentation to describe torus-2QoS. Date: Fri, 3 Sep 2010 10:43:07 -0600 Message-ID: <1283532194-27112-12-git-send-email-jaschut@sandia.gov> References: <1283532194-27112-1-git-send-email-jaschut@sandia.gov> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1283532194-27112-1-git-send-email-jaschut-4OHPYypu0djtX7QSmKvirg@public.gmane.org> Sender: linux-rdma-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Cc: sashak-smomgflXvOZWk0Htik3J/w@public.gmane.org, Jim Schutt List-Id: linux-rdma@vger.kernel.org Signed-off-by: Jim Schutt --- opensm/doc/current-routing.txt | 269 +++++++++++++++++++++++++++++++++++++++- opensm/man/opensm.8.in | 9 ++- 2 files changed, 275 insertions(+), 3 deletions(-) diff --git a/opensm/doc/current-routing.txt b/opensm/doc/current-routing.txt index 1302860..78a2e01 100644 --- a/opensm/doc/current-routing.txt +++ b/opensm/doc/current-routing.txt @@ -1,7 +1,7 @@ Current OpenSM Routing -7/9/07 +10/9/09 -OpenSM offers five routing engines: +OpenSM offers six routing engines: 1. Min Hop Algorithm - based on the minimum hops to each node where the path length is optimized. @@ -28,6 +28,13 @@ two switches. This provides deadlock free routes for hypercubes when the fabric is cabled as a hypercube and for meshes when cabled as a mesh (see details below). +6. Torus-2QoS unicast routing algorithm - a DOR-based routing algorithm +specialized for 2D/3D torus topologies. Torus-2QoS provides deadlock-free +routing while supporting two quality of service (QoS) levels. In addition +it is able to route around multiple failed fabric links or a single failed +fabric switch without introducing deadlocks, and without changing path SL +values granted before the failure. + OpenSM provides an optional unicast routing cache (enabled by -A or --ucast_cache options). When enabled, unicast routing cache prevents routing recalculation (which is a heavy task in a large cluster) when @@ -388,3 +395,261 @@ ports, one port on one end of the cable, and the other port on the other end, continuing along the mesh dimension. Use '-R dor' option to activate the DOR algorithm. + +Torus-2QoS Routing Algorithm +---------------------------- + +Torus-2QoS is routing algorithm designed for large-scale 2D/3D torus fabrics. +The torus-2QoS routing engine can provide the following functionality on +a 2D/3D torus: +- routing that is free of credit loops +- two levels of QoS, assuming switches support 8 data VLs +- ability to route around a single failed switch, and/or multiple failed + links, without + - introducing credit loops + - changing path SL values +- very short run times, with good scaling properties as fabric size + increases + +Torus-2QoS is a DOR-based algorithm that avoids deadlocks that would otherwise +occur in a torus using the concept of a dateline for each torus dimension. +It encodes into a path SL which datelines the path crosses as follows: + + sl = 0; + for (d = 0; d < torus_dimensions; d++) + /* path_crosses_dateline(d) returns 0 or 1 */ + sl |= path_crosses_dateline(d) << d; + +For a 3D torus, that leaves one SL bit free, which torus-2QoS uses to +implement two QoS levels. + +This is possible because torus-2QoS also makes use of the output port +dependence of the switch SL2VL maps. It computes in which torus coordinate +direction each interswitch link "points", and writes SL2VL maps for such +ports as follows: + + for (sl = 0; sl < 16; sl ++) + /* cdir(port) reports which torus coordinate direction a switch port + * "points" in, and returns 0, 1, or 2 */ + sl2vl(iport,oport,sl) = 0x1 & (sl >> cdir(oport)); + +Thus torus-2QoS consumes 8 SL values (SL bits 0-2) and 2 VL values (VL bit 0) +per QoS level to provide deadlock-free routing on a 3D torus. + +Torus-2QoS routes around link failure by "taking the long way around" any +1D ring interrupted by a link failure. For example, consider the 2D 6x5 +torus below, where switches are denoted by [+a-zA-Z]: + + | | | | | | + 4 --+----+----+----+----+----+-- + | | | | | | + 3 --+----+----+----D----+----+-- + | | | | | | + 2 --+----+----I----r----+----+-- + | | | | | | + 1 --m----S----n----T----o----p-- + | | | | | | + y=0 --+----+----+----+----+----+-- + | | | | | | + + x=0 1 2 3 4 5 + +For a pristine fabric the path from S to D would be S-n-T-r-d. In the +event that either link S-n or n-T has failed, torus-2QoS would use the path +S-m-p-o-T-r-D. Note that it can do this without changing the path SL +value; once the 1D ring m-S-n-T-o-p-m has been broken by failure, path +segments using it cannot contribute to deadlock, and the x-direction +dateline (between, say, x=5 and x=0) can be ignored for path segments on +that ring. + +One result of this is that torus-2QoS can route around many simultaneous +link failures, as long as no 1D ring is broken into disjoint regions. For +example, if links n-T and T-o have both failed, that ring has been broken +into two disjoint regions, T and o-p-m-S-n. Torus-2QoS checks for such +issues, reports if they are found, and refuses to route such fabrics. + +Handling a failed switch under DOR requires introducing into a path at +least one turn that would be otherwise "illegal", i.e. not allowed by DOR +rules. Torus-2QoS will introduce such a turn as close as possible to the +failed switch in order to route around it. + +In the above example, suppose switch T has failed, and consider the path +from S to D. Torus-2QoS will produce the path S-n-I-r-D, rather than the +S-n-T-r-D path for a pristine torus, by introducing an early turn at n. +For traffic arriving at switch I from n, normal DOR rules will generate an +illegal turn in the path from S to D at I, and a legal turn at r. + +Torus-2QoS will also use the input port dependence of SL2VL maps to set VL +bit 1 (which would be otherwise unused) for y-x, z-x, and z-y turns, i.e., +those turns that are illegal under DOR. This causes the first hop after +any such turn to use a separate set of VL values, and prevents deadlock in +the presence of a single failed switch. + +For any given path, only the hops after a turn that is illegal under DOR +can contribute to a credit loop that leads to deadlock. So in the example +above with failed switch T, the location of the illegal turn at I in the +path from S to D requires that any credit loop caused by that turn must +encircle the failed switch at T. Thus the second and later hops after the +illegal turn at I (i.e., hop r-D) cannot contribute to a credit loop +because they cannot be used to construct a loop encircling T. The hop I-r +uses a separate VL, so it cannot contribute to a credit loop encircling T. + +Extending this argument shows that in addition to being capable of routing +around a single switch failure without introducing deadlock, torus-2QoS can +also route around multiple failed switches on the condition they are +adjacent in the last dimension routed by DOR. For example, consider the +following case on a 6x6 2D torus: + + + | | | | | | + 5 --+----+----+----+----+----+-- + | | | | | | + 4 --+----+----+----D----+----+-- + | | | | | | + 3 --+----+----I----u----+----+-- + | | | | | | + 2 --+----+----q----R----+----+-- + | | | | | | + 1 --m----S----n----T----o----p-- + | | | | | | + y=0 --+----+----+----+----+----+-- + | | | | | | + + x=0 1 2 3 4 5 + + +Suppose switches T and R have failed, and consider the path from S to D. +Torus-2QoS will generate the path S-n-q-I-u-D, with an illegal turn at +switch I, and with hop I-u using a VL with bit 1 set. + +As a further example, consider a case that torus-2QoS cannot route without +deadlock: two failed switches adjacent in a dimension that is not the last +dimension routed by DOR; here the failed switches are O and T: + + | | | | | | + 5 --+----+----+----+----+----+-- + | | | | | | + 4 --+----+----+----+----+----+-- + | | | | | | + 3 --+----+----+----+----D----+-- + | | | | | | + 2 --+----+----I----q----r----+-- + | | | | | | + 1 --m----S----n----O----T----p-- + | | | | | | + y=0 --+----+----+----+----+----+-- + | | | | | | + + x=0 1 2 3 4 5 + +In a pristine fabric, torus-2QoS would generate the path from S to D as +S-n-O-T-r-D. With failed switches O and T, torus-2QoS will generate the +path S-n-I-q-r-D, with illegal turn at switch I, and with hop I-q using a +VL with bit 1 set. In contrast to the earlier examples, the second hop +after the illegal turn, q-r, can be used to construct a credit loop +encircling the failed switches. + +Since torus-2QoS uses all four available SL bits, and the three data VL +bits that are typically available in current switches, there is no way +to use SL/VL values to separate multicast traffic from unicast traffic. +Thus, torus-2QoS must generate multicast routing such that credit loops +cannot arise from a combination of multicast and unicast path segments. + +It turns out that it is possible to construct spanning trees for multicast +routing that have that property. For the 2D 6x5 torus example above, here +is the full-fabric spanning tree that torus-2QoS will construct, where "x" +is the root switch and each "+" is a non-root switch: + + 4 + + + + + + + | | | | | | + 3 + + + + + + + | | | | | | + 2 +----+----+----x----+----+ + | | | | | | + 1 + + + + + + + | | | | | | + y=0 + + + + + + + + x=0 1 2 3 4 5 + +For multicast traffic routed from root to tip, every turn in the above +spanning tree is a legal DOR turn. + +For traffic routed from tip to root, and some traffic routed through the +root, turns are not legal DOR turns. However, to construct a credit loop, +the union of multicast routing on this spanning tree with DOR unicast +routing can only provide 3 of the 4 turns needed for the loop. + +In addition, if none of the above spanning tree branches crosses a dateline +used for unicast credit loop avoidance on a torus, and if multicast traffic +is confined to SL 0 or SL 8 (recall that torus-2QoS uses SL bit 3 to +differentiate QoS level), then multicast traffic also cannot contribute to +the "ring" credit loops that are otherwise possible in a torus. + +Torus-2QoS uses these ideas to create a master spanning tree. Every +multicast group spanning tree will be constructed as a subset of the master +tree, with the same root as the master tree. + +Such multicast group spanning trees will in general not be optimal for +groups which are a subset of the full fabric. However, this compromise must +be made to enable support for two QoS levels on a torus while preventing +credit loops. + +In the presence of link or switch failures that result in a fabric for +which torus-2QoS can generate credit-loop-free unicast routes, it is also +possible to generate a master spanning tree for multicast that retains the +required properties. For example, consider that same 2D 6x5 torus, with +the link from (2,2) to (3,2) failed. Torus-2QoS will generate the following +master spanning tree: + + 4 + + + + + + + | | | | | | + 3 + + + + + + + | | | | | | + 2 --+----+----+ x----+----+-- + | | | | | | + 1 + + + + + + + | | | | | | + y=0 + + + + + + + + x=0 1 2 3 4 5 + +Two things are notable about this master spanning tree. First, assuming +the x dateline was between x=5 and x=0, this spanning tree has a branch +that crosses the dateline. However, just as for unicast, crossing a +dateline on a 1D ring (here, the ring for y=2) that is broken by a failure +cannot contribute to a torus credit loop. + +Second, this spanning tree is no longer optimal even for multicast groups +that encompass the entire fabric. That, unfortunately, is a compromise that +must be made to retain the other desirable properties of torus-2QoS routing. + +In the event that a single switch fails, torus-2QoS will generate a master +spanning tree that has no "extra" turns by appropriately selecting a root +switch. In the 2D 6x5 torus example, assume now that the switch at (3,2), +i.e. the root for a pristine fabric, fails. Torus-2QoS will generate the +following master spanning tree for that case: + + | + 4 + + + + + + + | | | | | | + 3 + + + + + + + | | | | | + 2 + + + + + + | | | | | + 1 +----+----x----+----+----+ + | | | | | | + y=0 + + + + + + + | + + x=0 1 2 3 4 5 + +Assuming the y dateline was between y=4 and y=0, this spanning tree has +a branch that crosses a dateline. However, again this cannot contribute +to credit loops as it occurs on a 1D ring (the ring for x=3) that is +broken by a failure, as in the above example. + +Due to the use made by torus-2QoS of SLs and VLs, QoS configuration should +only employ SL values 0 and 8, for both multicast and unicast. Also, +SL to VL map configuration must be under the complete control of torus-2QoS, +so any user-supplied configuration must and will be ignored. diff --git a/opensm/man/opensm.8.in b/opensm/man/opensm.8.in index 9053611..47dff99 100644 --- a/opensm/man/opensm.8.in +++ b/opensm/man/opensm.8.in @@ -649,7 +649,7 @@ compiling opensm with -DROUTER_EXP which has been obsoleted. .SH ROUTING .PP -OpenSM now offers five routing engines: +OpenSM now offers six routing engines: 1. Min Hop Algorithm - based on the minimum hops to each node where the path length is optimized. @@ -678,6 +678,13 @@ two switches. This provides deadlock free routes for hypercubes when the fabric is cabled as a hypercube and for meshes when cabled as a mesh (see details below). +6. Torus-2QoS unicast routing algorithm - a DOR-based routing algorithm +specialized for 2D/3D torus topologies. Torus-2QoS provides deadlock-free +routing while supporting two quality of service (QoS) levels. In addition +it is able to route around multiple failed fabric links or a single failed +fabric switch without introducing deadlocks, and without changing path SL +values granted before the failure. + OpenSM also supports a file method which can load routes from a table. See \'Modular Routing Engine\' for more information on this. -- 1.6.2.2 -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html