Aleksey Yeschenko created CASSANDRA-10989:
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Summary: Move away from SEDA to TPC
Key: CASSANDRA-10989
URL: https://issues.apache.org/jira/browse/CASSANDRA-10989
Project: Cassandra
Issue Type: Improvement
Reporter: Aleksey Yeschenko
Since its inception, Cassandra has been utilising [SEDA
|http://www.eecs.harvard.edu/~mdw/papers/seda-sosp01.pdf] at its core.
As originally conceived, it means every request is split into several stages,
and each stage is backed by a thread pool. That imposes certain challenges:
- thread parking/unparking overheads (partially improved by SEPExecutor in
CASSANDRA-4718)
- extensive context switching (i-/d- caches thrashing)
- less than optimal multiple writer/multiple reader data structures for
memtables, partitions, metrics, more
- hard to grok concurrent code
- large number of GC roots, longer TTSP
- increased complexity for moving data structures off java heap
- inability to easily balance writes/reads/compaction/flushing
Latency implications of SEDA have been acknowledged by the authors themselves -
see 2010 [retrospective on
SEDA|http://matt-welsh.blogspot.co.uk/2010/07/retrospective-on-seda.html].
To fix these issues (and more), two years ago at NGCC [~benedict] suggested
moving Cassandra away from SEDA to the more mechanically sympathetic thread per
core architecture (TPC). See the slides from the original presentation
[here|https://docs.google.com/presentation/d/19_U8I7mq9JKBjgPmmi6Hri3y308QEx1FmXLt-53QqEw/edit?ts=56265eb4#slide=id.g98ad32b25_1_19].
In a nutshell, each core would become a logical shared nothing micro instance
of Cassandra, taking over a portion of the node’s range {{*}}.
Client connections will be assigned randomly to one of the cores (sharing a
single listen socket). A request that cannot be served by the client’s core
will be proxied to the one owning the data, similar to the way we perform
remote coordination today.
Each thread (pinned to an exclusive core) would have a single event loop, and
be responsible for both serving requests and performing maintenance tasks
(flushing, compaction, repair), scheduling them intelligently.
One notable exception from the original proposal is that we cannot,
unfortunately, use linux AIO for file I/O, as it's only properly implemented
for xfs. We might, however, have a specialised implementation for xfs and
Windows (based on IOCP) later. In the meantime, we have no other choice other
than to hand off I/O that cannot be served from cache to a separate threadpool.
Transitioning from SEDA to TPC will be done in stages, incrementally and in
parallel.
This is a high-level overview meta-ticket that will track JIRA issues for each
individual stage.
{{*}} they’ll share certain things still, like schema, gossip, file I/O
threadpool(s), and maybe MessagingService.
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