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From "Joe McDonnell (Code Review)" <>
Subject [Impala-ASF-CR] IMPALA-1575: Yield admission control resources at query end
Date Thu, 03 Aug 2017 01:11:46 GMT
Joe McDonnell has uploaded a new patch set (#3).

Change subject: IMPALA-1575: Yield admission control resources at query end

IMPALA-1575: Yield admission control resources at query end

Currently, a query does not release admission control
resources until the client calls UnregisterQuery. Slow
clients can hold admission control resources even after
the query has reached a state where it will no longer
return any rows. Specifically, in the following
cases, the query is completed, but the client must
still call UnregisterQuery to release admission control

1. The query encounters an error and fails
2. The query is cancelled due to the idle query timeout
3. The query reaches eos (or the DML completes)
4. The client cancels the query without closing the query

This change releases admission control resources as
soon as the query reaches a state where it cannot
return any rows rather than waiting for the client
to explicitly end the query.

When cancelling a query, the coordinator asynchronously
notifies all fragment instances to cancel. The
coordinator does not wait for the fragment instances
to respond, so the cancel case can release admission
control resources while some fragment instances may
continue to run until the cancel takes effect. The
concern with this behavior is that the fragment
instances may continue to use memory and cause subsequent
admitted queries to fail. This is already possible today,
as a client can directly close a running query (which
cancels the query and unregisters the query immediately).
For example, the session idle timeout does this. However,
this change expands the circumstances where this can

Admission control based on mem_limit operates differently.
It relies on the reported memory usage of each
QueryState to generate a cumulative memory usage across
all of the instances. Admission control's behavior is
determined by when the QueryState releases its memory.

The existing behavior releases the query's memory on the
destruction of the QueryState, which occurs when the
query is unregistered. This matches the existing behavior
for admission control prior to this change. To support
the new behavior for mem_limit, the QueryState will now
release query resources when the last fragment instance
terminates. This unregisters the query memory tracker,
which results in the admission control memory resources
being freed. Backend tests do not spawn fragment instance
threads and are not subject to the new semantics for
releasing resources. Instead, the code detects this case
and releases resources in the old location.

To test both aspects of this change, the admission control
test (custom_cluster/ has
been modified to use four different modes of ending a
query: client cancelling a query, the query hitting an
idle timeout, the query reaching eos, and the client
closing the query. The test uses a mix of all four.
After the query ends, all clients wait for the test
to complete before closing the query or closing the
connection. This ensures that the admission control
decisions are based entirely on the query end behavior.
This test works for both query admission control
and mem_limit admission control.

Change-Id: Ia5003d017b3142a160bacf7e3569ff26026b1700
M be/src/runtime/
M be/src/runtime/coordinator.h
M be/src/runtime/
M be/src/runtime/
M be/src/runtime/query-state.h
M be/src/service/
M tests/custom_cluster/
7 files changed, 170 insertions(+), 105 deletions(-)

  git pull ssh:// refs/changes/79/7079/3
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Gerrit-MessageType: newpatchset
Gerrit-Change-Id: Ia5003d017b3142a160bacf7e3569ff26026b1700
Gerrit-PatchSet: 3
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Joe McDonnell <>
Gerrit-Reviewer: Dan Hecht <>
Gerrit-Reviewer: Joe McDonnell <>
Gerrit-Reviewer: Marcel Kornacker <>
Gerrit-Reviewer: Matthew Jacobs <>

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