why redis-cluster use 16384 slots?

[qunchenmy]

why redis-cluster use 16384 slots? crc16() can have 2^16 -1=65535 different remainders。

[antirez]

The reason is:

1. Normal heartbeat packets carry the full configuration of a node, that can be replaced in an idempotent way with the old in order to update an old config. This means they contain the slots configuration for a node, in raw form, that uses 2k of space with16k slots, but would use a prohibitive 8k of space using 65k slots.
2. At the same time it is unlikely that Redis Cluster would scale to more than 1000 mater nodes because of other design tradeoffs.

So 16k was in the right range to ensure enough slots per master with a max of 1000 maters, but a small enough number to propagate the slot configuration as a raw bitmap easily. Note that in small clusters the bitmap would be hard to compress because when N is small the bitmap would have slots/N bits set that is a large percentage of bits set.

[exceptionplayer]

Hey,i have two questions about you answer:

1. why 8K of space is prohibitive using 65 slots?
2. why the bitmap should be compressed?

[mind4s]

@medusar

1. not 65, that's 65k = 8 * 8 (8 bit/byte) * 1024(1k) = 8K bitmap size
2. Compress bitmap to reduce network traffic

[exceptionplayer]

@foojolt thank you very much

[courage007]

why:
16k was in the right range to ensure enough slots per master with a max of 1000 maters.
what is the logic in the background？

[guoruibiao]

thx. The balance of bitmap's size and number of master is the key.

[qfl19911216]

what is other design tradeoffs? @foojolt

[qfl19911216]

8K bitmap size means that bitmap size is 8 bits? @foojolt

[qfl19911216]

How to understand the sentence not 65, that's 65K = 8 * 8 (8 bit/byte) * 1024 (1k) = 8K bitmap size?@foojolt

[haiyuzhu]

How to understand the sentence not 65, that's 65K = 8 * 8 (8 bit/byte) * 1024 (1k) = 8K bitmap size?@foojolt

crc16() can have 2^16 -1=65535, which means the bitmap has 65535 bits. so the size of the bitmap can be calculated by 65535 / 8 (8bit/byte)/1024(1k)=7.99 Kbytes.

[cqlxcnp]

Thanks for answers

[shao-h]

How to understand the sentence not 65, that's 65K = 8 * 8 (8 bit/byte) * 1024 (1k) = 8K bitmap size?@foojolt

crc16() can have 2^16 -1=65535, which means the bitmap has 65535 bits. so the size of the bitmap can be calculated by 65535 / 8 (8bit/byte)/1024(1k)=7.99 Kbytes.

Hi, I didn't find where to compress the slots bitmap in source code, did I miss something?

[trevor211]

How to understand the sentence not 65, that's 65K = 8 * 8 (8 bit/byte) * 1024 (1k) = 8K bitmap size?@foojolt

crc16() can have 2^16 -1=65535, which means the bitmap has 65535 bits. so the size of the bitmap can be calculated by 65535 / 8 (8bit/byte)/1024(1k)=7.99 Kbytes.

Hi, I didn't find where to compress the slots bitmap in source code, did I miss something?

No compression, just 1 bit for 1 slot.
See the code: https://github.com/antirez/redis/blob/ac441c741379dd4002f664c81047e8412cb793d0/src/cluster.h#L117

[sjclijie]

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