hash-zipmap-max-entries and HVALS

[Tadeusz Wójcik]

Hi,

When using hash-zipmap-max-entries with large value such as 3000-5000
does it make big difference in performance when using only HVALS and
HSET/HMSET commands with hash structure? HVALS is normally O(N), and
when using it with hash-zipmap-max-entries=5000 is it still O(N) or
O(N2)?
Thanks,
Tadek

[Didier Spezia]

Hi,

the zipmap data structure is described in the comments of:

https://github.com/antirez/redis/blob/2.4/src/zipmap.c

HVALS is still O(N)

Contrary to a normal hash, zipmap'd HSET is O(N), with a rather high

constant factor when the zipmap must be reallocated and copied.

The higher hash-zipmap-max-entries, the more expensive reallocation/copy

operations will be.

Since only one byte is used to store the size, complexity of zipmap'd

HLEN is also O(N) when N > 253 (while it is constant time for normal hash).

I don't not why. Personally, I would have put at least 2 bytes, to make sure

HLEN is kept constant time for the default size value (512).

Regards,

Didier.

[Tadeusz Wójcik]

Thanks @Didier, you confirmed my suspicions how it works,