Eiichi Sato created HBASE-17072:
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Summary: CPU usage starts to climb up to 90-100% when using G1GC
Key: HBASE-17072
URL: https://issues.apache.org/jira/browse/HBASE-17072
Project: HBase
Issue Type: Bug
Components: Performance, regionserver
Affects Versions: 1.2.0, 1.0.0
Reporter: Eiichi Sato
h5. Problem
CPU usage of a region server in our CDH 5.4.5 cluster, at some point, starts to
gradually get higher up to nearly 90-100% when using G1GC. We've also run into
this problem on CDH 5.7.3 and CDH 5.8.2.
In our production cluster, it normally takes a few weeks for this to happen
after restarting a RS. We reproduced this on our test cluster and attached the
results. Please note that, to make it easy to reproduce, we did some
"anti-tuning" on a table when running tests.
In metrics.png, soon after we started running some workloads against a test
cluster (CDH 5.8.2) at about 7 p.m. CPU usage of the two RSs started to rise.
Flame Graphs (slave1.svg to slave4.svg) are generated from jstack dumps of each
RS process around 10:30 a.m. the next day.
After investigating heapdumps from another occurrence on a test cluster running
CDH 5.7.3, we found that the ThreadLocalMap contain a lot of contiguous entries
of {{HFileBlock$PrefetchedHeader}} probably due to primary clustering. This
caused more loops in {{ThreadLocalMap#expungeStaleEntries()}}, consuming a
certain amount of CPU time. What is worse is that the method is called from
RPC metrics code, which means even a small amount of per-RPC time soon adds up
to a huge amount of CPU time.
This is very similar to the issue in HBASE-16616, but we have many
{{HFileBlock$PrefetchedHeader}} not only {{Counter$IndexHolder}} instances.
Here are some OQL counts from Eclipse Memory Analyzer (MAT). This shows a
number of ThreadLocal instances in the ThreadLocalMap of a single handler
thread.
{code}
SELECT *
FROM OBJECTS (SELECT AS RETAINED SET OBJECTS value
FROM OBJECTS 0x4ee380430) obj
WHERE obj.@clazz.@name =
"org.apache.hadoop.hbase.io.hfile.HFileBlock$PrefetchedHeader"
#=> 10980 instances
{code}
{code}
SELECT *
FROM OBJECTS (SELECT AS RETAINED SET OBJECTS value
FROM OBJECTS 0x4ee380430) obj
WHERE obj.@clazz.@name = "org.apache.hadoop.hbase.util.Counter$IndexHolder"
#=> 2052 instances
{code}
Although as described in HBASE-16616 this somewhat seems to be an issue in G1GC
side regarding weakly-reachable objects, we should keep ThreadLocal usage
minimal and avoid creating an indefinite number (in this case, a number of
HFiles) of ThreadLocal instances.
HBASE-16146 removes ThreadLocals from the RPC metrics code. That may solve the
issue (I just saw the patch, never tested it at all), but the
{{HFileBlock$PrefetchedHeader}} are still there in the ThreadLocalMap, which
may cause issues in the future again.
h5. Our Solution
We simply removed the whole {{HFileBlock$PrefetchedHeader}} caching and
fortunately we didn't notice any performance degradation for our production
workloads.
Because the PrefetchedHeader caching uses ThreadLocal and because RPCs are
handled randomly in any of the handlers, small Get or small Scan RPCs do not
benefit from the caching (See HBASE-10676 and HBASE-11402 for the details).
Probably, we need to see how well reads are saved by the caching for large Scan
or Get RPCs and especially for compactions if we really remove the caching.
It's probably better if we can remove ThreadLocals without breaking the current
caching behavior.
FWIW, I'm attaching the patch we applied. It's for CDH 5.4.5.
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