Michael Semb Wever created CASSANDRA-15922:
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Summary: High CAS failures in NativeAllocator.Region.allocate(..)
Key: CASSANDRA-15922
URL: https://issues.apache.org/jira/browse/CASSANDRA-15922
Project: Cassandra
Issue Type: Bug
Components: Local/Memtable
Reporter: Michael Semb Wever
Assignee: Michael Semb Wever
Attachments: NativeAllocatorRegionTest.java, Screen Shot 2020-07-05 at
13.16.10.png, Screen Shot 2020-07-05 at 13.26.17.png, Screen Shot 2020-07-05 at
13.35.55.png, Screen Shot 2020-07-05 at 13.37.01.png, Screen Shot 2020-07-05 at
13.48.16.png, profile_pbdpc23zafsrh_20200702.svg
h6. Problem
The method {{NativeAllocator.Region.allocate(..)}} uses an {{AtomicInteger}}
for the current offset in the region. Allocations depends on a
{{.compareAndSet(..)}} call.
In highly contended environments the CAS failures can be high, starving writes
in a running Cassandra node.
h6. Example
CPU time of 33% stuck in the {{NativeAllocator.Region.allocate(..)}} loop, from
the CAS failures, in nodes during heavy spark analytics write load.
These nodes: 40 CPU cores and 256GB ram; have relevant settings
- {{memtable_allocation_type: offheap_objects}}
- {{memtable_offheap_space_in_mb: 5120}}
- {{concurrent_writes: 160}}
Numerous flamegraphs demonstrate the problem. See attached.
h6. Suggestion: ThreadLocal Regions
One possible solution is to have separate Regions per thread.
Code wise this is relatively easy to do, for example replacing
NativeAllocator:59
{code}private final AtomicReference<Region> currentRegion = new
AtomicReference<>();{code}
with
{code}private final ThreadLocal<AtomicReference<Region>> currentRegion = new
AtomicReference<>() {...};{code}
But this approach substantially changes the allocation behaviour, with more
than concurrent_writes number of Regions in use at any one time.
h6. Suggestion: Simple Contention Management Algorithm (Constant Backoff)
Another possible solution is to introduce a contention management algorithm
that a) reduces CAS failures in high contention environments, b) doesn't impact
normal environments, and c) keeps the allocation strategy of using one region
at a time.
The research paper [arXiv:1305.5800|https://arxiv.org/abs/1305.5800] describes
this contention CAS problem and demonstrates a number of algorithms to apply.
The simplest of these algorithms is the Constant Backoff CAS Algorithm.
Applying the Constant Backoff CAS Algorithm involves adding one line of code to
{{NativeAllocator.Region.allocate(..)}} to sleep for one (or some constant
number) nanoseconds after a CAS failure occurs.
That is...
{code}
// we raced and lost alloc, try again
LockSupport.parkNanos(1);
{code}
h6. Constant Backoff CAS Algorithm Experiments
Using the code attached in NativeAllocatorRegionTest.java the concurrency and
CAS failures of {{NativeAllocator.Region.allocate(..)}} can be demonstrated.
In the attached {{NativeAllocatorRegionTest}} class, which can be run
standalone, the {{Region}} class: copied from {{NativeAllocator.Region}}; has
also the {{casFailures}} field added. The following two screenshots are from
data collected from this class on a 6 CPU (12 core) MBP, running the
{{NativeAllocatorRegionTest.testRegionCAS}} method.
This attached screenshot shows the number of CAS failures during the life of a
Region (over ~215 millions allocations), using different threads and park
times. This illustrates the improvement (reduction) of CAS failures from zero
park time, through orders of magnitude, up to 10000000ns (10ms). The biggest
improvement is from no algorithm to a sleep of 1ns where CAS failures are ~two
orders of magnitude lower. From 10μs there is a significant drop also at low
contention rates.
This attached screenshot shows the time it takes to fill a Region (~215
millions allocations), using different threads and park times. The biggest
improvement is from no algorithm to a sleep of 1ns where performance is one
orders of magnitude faster. From 100μs there is a even further significant
drop, especially at low contention rates.
Repeating the test run show reliably similar results: screenshot1 and
screenshot2.
h6. Region Per Thread Experiments
Implementing Region Per Thread: see the
{{NativeAllocatorRegionTest.testRegionThreadLocal}} method; we can expect zero
CAS failures of the life of a Region. For performance we see two orders of
magnitude lower times to fill up the Region (~420ms).
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