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From "Ariel Weisberg (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7937) Apply backpressure gently when overloaded with writes
Date Mon, 13 Jun 2016 21:51:04 GMT


Ariel Weisberg commented on CASSANDRA-7937:

I think we can make this situation better, and I mentioned some ideas at NGCC and in CASSANDRA-11327.

There are two issues.

The first is that if flushing falls behind throughput falls to zero instead of progressing
at the rate at which flushing progresses which is usually not zero. Right now it looks like
it is zero because flushing doesn't release any memory as it progresses and is all or nothing.

Aleksey mentioned we could do something like early opening for flushing so that memory is
made available sooner. Alternatively we could overcommit and then gradually release memory
as flushing progresses.

The second is that flushing falls behind in several reasonable configurations. Ingest has
gotten faster and I don't think flushing has as much so it's easier for it to fall behind
if it's driven by a single thread against a busy device (even a fast SSD). I haven't tested
this yet, but I suspect that if you use multiple JBOD paths for a fast device like an SSD
and increase memtable_flush_writers you will get enough additional flushing throughput to
keep up with ingest. Right now flushing is single threaded for a single path and only one
flush can occur at any time.

Flushing falling behind is more noticeable when you let compaction have more threads and a
bigger rate limit because it can dirty enough memory in the filesystem cache that when it
flushes it causes a temporally localized slowdown in flushing that is enough to cause timeouts
when there is no more free memory because flushing didn't finish soon enough.

I think the long term solution is that the further flushing falls behind the more concurrent
flush threads we start deploying kind of like compaction up to the configured limit. [Right
now there is a single thread scheduling them and waiting on the result.|]
memtable_flush_writers doesn't help due to this code here that only [generates more flush
runnables for a memtable if there are multiple directories|].
C* is already divvying up the heap using memtable_cleanup_threshold which would allow for
concurrent flushing it's just not actually flushing concurrently.

> Apply backpressure gently when overloaded with writes
> -----------------------------------------------------
>                 Key: CASSANDRA-7937
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>         Environment: Cassandra 2.0
>            Reporter: Piotr Kołaczkowski
>              Labels: performance
> When writing huge amounts of data into C* cluster from analytic tools like Hadoop or
Apache Spark, we can see that often C* can't keep up with the load. This is because analytic
tools typically write data "as fast as they can" in parallel, from many nodes and they are
not artificially rate-limited, so C* is the bottleneck here. Also, increasing the number of
nodes doesn't really help, because in a collocated setup this also increases number of Hadoop/Spark
nodes (writers) and although possible write performance is higher, the problem still remains.
> We observe the following behavior:
> 1. data is ingested at an extreme fast pace into memtables and flush queue fills up
> 2. the available memory limit for memtables is reached and writes are no longer accepted
> 3. the application gets hit by "write timeout", and retries repeatedly, in vain 
> 4. after several failed attempts to write, the job gets aborted 
> Desired behaviour:
> 1. data is ingested at an extreme fast pace into memtables and flush queue fills up
> 2. after exceeding some memtable "fill threshold", C* applies adaptive rate limiting
to writes - the more the buffers are filled-up, the less writes/s are accepted, however writes
still occur within the write timeout.
> 3. thanks to slowed down data ingestion, now flush can finish before all the memory gets
> Of course the details how rate limiting could be done are up for a discussion.
> It may be also worth considering putting such logic into the driver, not C* core, but
then C* needs to expose at least the following information to the driver, so we could calculate
the desired maximum data rate:
> 1. current amount of memory available for writes before they would completely block
> 2. total amount of data queued to be flushed and flush progress (amount of data to flush
remaining for the memtable currently being flushed)
> 3. average flush write speed

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