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https://issues.apache.org/jira/browse/CASSANDRA-15229?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
ZhaoYang updated CASSANDRA-15229:
---------------------------------
Description:
The BufferPool was never intended to be used for a {{ChunkCache}}, and we need
to either change our behaviour to handle uncorrelated lifetimes or use
something else. This is particularly important with the default chunk size for
compressed sstables being reduced. If we address the problem, we should also
utilise the BufferPool for native transport connections like we do for
internode messaging, and reduce the number of pooling solutions we employ.
Probably the best thing to do is to improve BufferPool’s behaviour when used
for things with uncorrelated lifetimes, which essentially boils down to
tracking those chunks that have not been freed and re-circulating them when we
run out of completely free blocks. We should probably also permit
instantiating separate {{BufferPool}}, so that we can insulate internode
messaging from the {{ChunkCache}}, or at least have separate memory bounds for
each, and only share fully-freed chunks.
With these improvements we can also safely increase the {{BufferPool}} chunk
size to 128KiB or 256KiB, to guarantee we can fit compressed pages and reduce
the amount of global coordination and per-allocation overhead. We don’t need
1KiB granularity for allocations, nor 16 byte granularity for tiny allocations.
-----
Since CASSANDRA-5863, chunk cache is implemented to use buffer pool. When local
pool is full, one of its chunks will be evicted and only put back to global
pool when all buffers in the evicted chunk are released. But due to chunk
cache, buffers can be held for long period of time, preventing evicted chunk to
be recycled even though most of space in the evicted chunk are free.
There two things need to be improved:
1. Evicted chunk with free space should be recycled to global pool, even if
it's not fully free. It's doable in 4.0.
2. Reduce fragmentation caused by different buffer size. With #1, partially
freed chunk will be available for allocation, but "holes" in the partially
freed chunk are with different sizes. We should consider allocating fixed
buffer size which is unlikely to fit in 4.0.
was:
The BufferPool was never intended to be used for a {{ChunkCache}}, and we need
to either change our behaviour to handle uncorrelated lifetimes or use
something else. This is particularly important with the default chunk size for
compressed sstables being reduced. If we address the problem, we should also
utilise the BufferPool for native transport connections like we do for
internode messaging, and reduce the number of pooling solutions we employ.
Probably the best thing to do is to improve BufferPool’s behaviour when used
for things with uncorrelated lifetimes, which essentially boils down to
tracking those chunks that have not been freed and re-circulating them when we
run out of completely free blocks. We should probably also permit
instantiating separate {{BufferPool}}, so that we can insulate internode
messaging from the {{ChunkCache}}, or at least have separate memory bounds for
each, and only share fully-freed chunks.
With these improvements we can also safely increase the {{BufferPool}} chunk
size to 128KiB or 256KiB, to guarantee we can fit compressed pages and reduce
the amount of global coordination and per-allocation overhead. We don’t need
1KiB granularity for allocations, nor 16 byte granularity for tiny allocations.
> BufferPool Regression
> ---------------------
>
> Key: CASSANDRA-15229
> URL: https://issues.apache.org/jira/browse/CASSANDRA-15229
> Project: Cassandra
> Issue Type: Bug
> Components: Local/Caching
> Reporter: Benedict Elliott Smith
> Assignee: ZhaoYang
> Priority: Normal
> Fix For: 4.0, 4.0-beta
>
> Attachments: 15229-count.png, 15229-direct.png, 15229-hit-rate.png,
> 15229-recirculate-count.png, 15229-recirculate-hit-rate.png,
> 15229-recirculate-size.png, 15229-recirculate.png, 15229-size.png,
> 15229-unsafe.png
>
>
> The BufferPool was never intended to be used for a {{ChunkCache}}, and we
> need to either change our behaviour to handle uncorrelated lifetimes or use
> something else. This is particularly important with the default chunk size
> for compressed sstables being reduced. If we address the problem, we should
> also utilise the BufferPool for native transport connections like we do for
> internode messaging, and reduce the number of pooling solutions we employ.
> Probably the best thing to do is to improve BufferPool’s behaviour when used
> for things with uncorrelated lifetimes, which essentially boils down to
> tracking those chunks that have not been freed and re-circulating them when
> we run out of completely free blocks. We should probably also permit
> instantiating separate {{BufferPool}}, so that we can insulate internode
> messaging from the {{ChunkCache}}, or at least have separate memory bounds
> for each, and only share fully-freed chunks.
> With these improvements we can also safely increase the {{BufferPool}} chunk
> size to 128KiB or 256KiB, to guarantee we can fit compressed pages and reduce
> the amount of global coordination and per-allocation overhead. We don’t need
> 1KiB granularity for allocations, nor 16 byte granularity for tiny
> allocations.
> -----
> Since CASSANDRA-5863, chunk cache is implemented to use buffer pool. When
> local pool is full, one of its chunks will be evicted and only put back to
> global pool when all buffers in the evicted chunk are released. But due to
> chunk cache, buffers can be held for long period of time, preventing evicted
> chunk to be recycled even though most of space in the evicted chunk are free.
> There two things need to be improved:
> 1. Evicted chunk with free space should be recycled to global pool, even if
> it's not fully free. It's doable in 4.0.
> 2. Reduce fragmentation caused by different buffer size. With #1, partially
> freed chunk will be available for allocation, but "holes" in the partially
> freed chunk are with different sizes. We should consider allocating fixed
> buffer size which is unlikely to fit in 4.0.
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