[Impala-ASF-CR] IMPALA-6121: remove I/O mgr request context cache

["Tim Armstrong (Code Review)" <ge...@cloudera.org>]

Hello Michael Ho, 

I'd like you to reexamine a change. Please visit

    http://gerrit.cloudera.org:8080/8408

to look at the new patch set (#4).

Change subject: IMPALA-6121: remove I/O mgr request context cache
......................................................................

IMPALA-6121: remove I/O mgr request context cache

This simplifies the lifecycle of the request contexts and eliminates
some code. The comments claim that request context cache improves
performance when allocating smallish the objects. But allocating
from TCMalloc's thread caches should scale much better than a
global object pool protected by a lock.

I needed to move the definition to a non-internal header file so that it
was visible to clients that manage it by unique_ptr.

We also do not need to transfer the request contexts to the RuntimeState
since I/O buffers do not leave scanners now.

Testing:
Ran exhaustive tests.

Change-Id: I91414eceaa4938fccd74686fe6bebede6ef36108
---
M be/src/exec/hdfs-scan-node-base.cc
M be/src/exec/hdfs-scan-node-base.h
M be/src/exec/hdfs-scan-node-mt.cc
M be/src/exec/hdfs-scan-node.cc
M be/src/runtime/disk-io-mgr-internal.h
M be/src/runtime/disk-io-mgr-reader-context.cc
A be/src/runtime/disk-io-mgr-reader-context.h
M be/src/runtime/disk-io-mgr-stress.cc
M be/src/runtime/disk-io-mgr-test.cc
M be/src/runtime/disk-io-mgr.cc
M be/src/runtime/disk-io-mgr.h
M be/src/runtime/runtime-state.cc
M be/src/runtime/runtime-state.h
M be/src/runtime/tmp-file-mgr-test.cc
M be/src/runtime/tmp-file-mgr.cc
M be/src/runtime/tmp-file-mgr.h
16 files changed, 614 insertions(+), 780 deletions(-)


  git pull ssh://gerrit.cloudera.org:29418/Impala-ASF refs/changes/08/8408/4
-- 
To view, visit http://gerrit.cloudera.org:8080/8408
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Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-MessageType: newpatchset
Gerrit-Change-Id: I91414eceaa4938fccd74686fe6bebede6ef36108
Gerrit-Change-Number: 8408
Gerrit-PatchSet: 4
Gerrit-Owner: Tim Armstrong <ta...@cloudera.com>
Gerrit-Reviewer: Michael Ho <kw...@cloudera.com>
Gerrit-Reviewer: Tim Armstrong <ta...@cloudera.com>