[kudu-CR] log: refactor log reading into an iterator-like interface

["Todd Lipcon (Code Review)" <ge...@cloudera.org>]

Hello Kudu Jenkins,

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

    http://gerrit.cloudera.org:8080/3038

to look at the new patch set (#2).

Change subject: log: refactor log reading into an iterator-like interface
......................................................................

log: refactor log reading into an iterator-like interface

This adds a new interface to iterate over a WAL one entry at a time. This
interface is both more natural to use and more performant: by not materializing
lots of entries in memory at the same time, the allocator thread cache should
be kept much hotter and a smaller amount of memory can be reused, keeping CPU
caches hotter.

Currently, this is just a refactor of the internals, with the existing
ReadEntries() method changed to use the iterator interface. Follow-on
patches will change call sites to use the iterator instead.

Change-Id: I5727951664600b7a591a6c892ea1e62469ceb109
---
M src/kudu/consensus/log_util.cc
M src/kudu/consensus/log_util.h
2 files changed, 245 insertions(+), 137 deletions(-)


  git pull ssh://gerrit.cloudera.org:29418/kudu refs/changes/38/3038/2
-- 
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Gerrit-MessageType: newpatchset
Gerrit-Change-Id: I5727951664600b7a591a6c892ea1e62469ceb109
Gerrit-PatchSet: 2
Gerrit-Project: kudu
Gerrit-Branch: master
Gerrit-Owner: Todd Lipcon <to...@apache.org>
Gerrit-Reviewer: Kudu Jenkins