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Add Scheduling Topology Hints #180

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merged 9 commits into from
Nov 25, 2021
Merged

Add Scheduling Topology Hints #180

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e3bc10c
add scheduling topology hint and extensive testing
csegarragonz Nov 24, 2021
b9c12e5
adding a couple more tests
csegarragonz Nov 24, 2021
0402cef
move tests to scheduler folder and change tags
csegarragonz Nov 24, 2021
850801e
move to callFunctions and remove publicMakeSchedulingDecision
csegarragonz Nov 24, 2021
941804e
add check for the recorded messages
csegarragonz Nov 24, 2021
1a91cfc
refactor hint from PAIRS to NEVER_ALONE
csegarragonz Nov 24, 2021
79862ab
set force local as a topology hint
csegarragonz Nov 24, 2021
93fedb7
change overloading logic as discussed offline
csegarragonz Nov 24, 2021
a325bc6
pr comments
csegarragonz Nov 25, 2021
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12 changes: 10 additions & 2 deletions include/faabric/scheduler/Scheduler.h
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Original file line number Diff line number Diff line change
Expand Up @@ -102,7 +102,9 @@ class Scheduler

faabric::util::SchedulingDecision callFunctions(
std::shared_ptr<faabric::BatchExecuteRequest> req,
bool forceLocal = false);
bool forceLocal = false,
faabric::util::SchedulingTopologyHint =
faabric::util::SchedulingTopologyHint::NORMAL);
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faabric::util::SchedulingDecision callFunctions(
std::shared_ptr<faabric::BatchExecuteRequest> req,
Expand Down Expand Up @@ -177,6 +179,11 @@ class Scheduler

void clearRecordedMessages();

faabric::util::SchedulingDecision publicMakeSchedulingDecision(
std::shared_ptr<faabric::BatchExecuteRequest> req,
bool forceLocal,
faabric::util::SchedulingTopologyHint topologyHint);
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@Shillaker Shillaker Nov 24, 2021 •
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The need for this function isn't immediately obvious and it feels like a bit of a hack. In general if an API needs to be changed to support a test you have one of two things going on: (i) your test is too invasive and checking too much internal logic; (ii) the API doesn't expose enough information.

In this case I think it's (i). I think it can be changed relatively easily as this and callFunctions have the same signature. It might be possible to change the tests to call callFunctions instead (as they have mock mode turned on). You can then add a check to make sure the underlying function calls have been disaptched to the expected hosts.

If this isn't possible, then we need to work out what's happening in callFunctions that doesn't work properly in mock mode.

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@csegarragonz csegarragonz Nov 24, 2021 •
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Yes, switching to callFunctions is pretty easy with the only caveat that we need to use the TestExecutor and TestExecutorFactory classes that currently lived in ./tests/tests/scheduler/test_executor.cpp.

I have moved the declaration of these classes to fixtures.h and kept the definition where it is.

I also add a check for the recorded messages in the function call client.


// ----------------------------------
// Exec graph
// ----------------------------------
Expand Down Expand Up @@ -233,7 +240,8 @@ class Scheduler

faabric::util::SchedulingDecision makeSchedulingDecision(
std::shared_ptr<faabric::BatchExecuteRequest> req,
bool forceLocal);
bool forceLocal,
faabric::util::SchedulingTopologyHint topologyHint);

faabric::util::SchedulingDecision doCallFunctions(
std::shared_ptr<faabric::BatchExecuteRequest> req,
Expand Down
12 changes: 12 additions & 0 deletions include/faabric/util/scheduling.h
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Expand Up @@ -41,4 +41,16 @@ class SchedulingDecision
int32_t appIdx,
int32_t groupIdx);
};

// Scheduling topology hints help the scheduler decide which host to assign new
// requests in a batch.
// - NORMAL: bin-packs requests to slots in hosts starting from the master
// host, and overloadds the master if it runs out of resources.
// - PAIRS: never allocates a single (non-master) request to a host without
// other requests of the batch.
enum SchedulingTopologyHint
{
NORMAL,
PAIRS
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};
}
58 changes: 47 additions & 11 deletions src/scheduler/Scheduler.cpp
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Expand Up @@ -210,7 +210,8 @@ void Scheduler::notifyExecutorShutdown(Executor* exec,

faabric::util::SchedulingDecision Scheduler::callFunctions(
std::shared_ptr<faabric::BatchExecuteRequest> req,
bool forceLocal)
bool forceLocal,
faabric::util::SchedulingTopologyHint topologyHint)
{
// Note, we assume all the messages are for the same function and have the
// same master host
Expand All @@ -236,7 +237,8 @@ faabric::util::SchedulingDecision Scheduler::callFunctions(

faabric::util::FullLock lock(mx);

SchedulingDecision decision = makeSchedulingDecision(req, forceLocal);
SchedulingDecision decision =
makeSchedulingDecision(req, forceLocal, topologyHint);

// Send out point-to-point mappings if necessary (unless being forced to
// execute locally, in which case they will be transmitted from the
Expand All @@ -249,9 +251,22 @@ faabric::util::SchedulingDecision Scheduler::callFunctions(
return doCallFunctions(req, decision, lock);
}

faabric::util::SchedulingDecision Scheduler::publicMakeSchedulingDecision(
std::shared_ptr<faabric::BatchExecuteRequest> req,
bool forceLocal,
faabric::util::SchedulingTopologyHint topologyHint)
{
if (!faabric::util::isTestMode()) {
throw std::runtime_error("This function must only be called in tests");
}

return makeSchedulingDecision(req, forceLocal, topologyHint);
}

faabric::util::SchedulingDecision Scheduler::makeSchedulingDecision(
std::shared_ptr<faabric::BatchExecuteRequest> req,
bool forceLocal)
bool forceLocal,
faabric::util::SchedulingTopologyHint topologyHint)
{
int nMessages = req->messages_size();
faabric::Message& firstMsg = req->mutable_messages()->at(0);
Expand Down Expand Up @@ -296,8 +311,20 @@ faabric::util::SchedulingDecision Scheduler::makeSchedulingDecision(
int available = r.slots() - r.usedslots();
int nOnThisHost = std::min(available, remainder);

for (int i = 0; i < nOnThisHost; i++) {
hosts.push_back(h);
// Under the pairs topology hint, we never allocate a single
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@Shillaker Shillaker Nov 24, 2021 •
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My gut here would instead be:

if(topologyHint == faabric::util::SchedulingTopologyHint::PAIRS &&
      nOnThisHost < 2) {
    // Move on if we can't colocate function with at least one other
    continue;
}

I think this fixes the issue you mention in the PR description too.

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@csegarragonz csegarragonz Nov 24, 2021 •
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I don't think this would behave as expected. For example:

  • We have 4 hosts with 4 slots each.
  • We want to schedule 9 requests (with the NEVER_ALONE hint).
  • We expect 4 requests scheduled to the first host, and 5 scheduled to the second.

However, I think your solution would schedule 5 requests on the first host and 4 on the second, as it would exhaust all possible hosts (nOnThisHost == 1 for all of them), and resort to overload the master.

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After an offline discussion, I use this change together with a change in the overloading logic that makes the issue mentioned in the description disappear.

// non-master request (id != 0) to a host without other
// requests of the batch
bool stickToPreviousHost =
(topologyHint ==
faabric::util::SchedulingTopologyHint::PAIRS &&
nOnThisHost == 1 && hosts.size() > 0);

if (stickToPreviousHost) {
hosts.push_back(hosts.back());
} else {
for (int i = 0; i < nOnThisHost; i++) {
hosts.push_back(h);
}
}

remainder -= nOnThisHost;
Expand All @@ -323,13 +350,22 @@ faabric::util::SchedulingDecision Scheduler::makeSchedulingDecision(
int available = r.slots() - r.usedslots();
int nOnThisHost = std::min(available, remainder);

// Register the host if it's exected a function
if (nOnThisHost > 0) {
registeredHosts[funcStr].insert(h);
}
bool stickToPreviousHost =
(topologyHint ==
faabric::util::SchedulingTopologyHint::PAIRS &&
nOnThisHost == 1 && hosts.size() > 0);
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for (int i = 0; i < nOnThisHost; i++) {
hosts.push_back(h);
if (stickToPreviousHost) {
hosts.push_back(hosts.back());
} else {
// Register the host if it's exected a function
if (nOnThisHost > 0) {
registeredHosts[funcStr].insert(h);
}

for (int i = 0; i < nOnThisHost; i++) {
hosts.push_back(h);
}
}

remainder -= nOnThisHost;
Expand Down
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