A sparse bitmap
Five thousand bits set across a million offsets, where a set is far smaller
highest-bit: 1000000 set-bits: 5000
Redis Bitmap Memory Calculator
Give the highest bit offset you will set and see what it costs. Redis strings are not sparse, so the highest offset decides the allocation whatever the density.
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Examples
Real input you can load into the tool above. Each one shows a different thing going wrong, because that is what the tool is for.
Daily active users
One bitmap per day for a month, at a realistic density
highest-bit: 1000000 set-bits: 400000 keys: 30
Common mistakes
These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.
Using a bitmap for sparse ids
The allocation is decided by the highest id, so one user with id 50 million costs 6 MB on its own.
Instead:Use a set below roughly 1 in 64 density, or remap ids to a dense range.
Assuming SETBIT is cheap on a new key
It allocates the whole string up to that offset in one go, which on a large offset is a noticeable pause on the main thread.
Instead:Pre-size deliberately, or avoid very large offsets.
Forgetting BITCOUNT is O(N)
It scans the whole string. On a 100 MB bitmap that blocks the server for the duration.
Instead:Use the range form, or keep a counter alongside.
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