redis no eviction mode, when will command be rejec...

Yes — with maxmemory-policy noeviction, Redis will start returning OOM errors once used_memory is above the configured maxmemory, but it does not perfectly prevent a single accepted command from pushing memory over the limit.

Redis checks memory when a client runs a command that adds data; if memory is over the limit, it applies the configured policy. With noeviction, Redis does not evict keys and instead returns an error for commands that would cache/add new data; read-only commands still work normally. (Redis)

So the practical behavior is:

1. If Redis is currently below maxmemory:
   A write command may be accepted. If that command allocates a lot of memory, it can push Redis over maxmemory. Redis docs explicitly note that a large command can temporarily exceed the limit by a large amount. (Redis)

2. After Redis is over maxmemory:
   New commands marked denyoom are rejected with an OOM error. Redis command metadata defines denyoom as: command is rejected when server memory usage is too high. Examples include memory-growing writes like SET, MSET, LPUSH, ZADD, XADD, etc. (Redis)

3. Not everything is rejected:
   Reads like GET, MGET, LRANGE, and admin/introspection commands generally continue. Memory-reducing commands such as DEL, UNLINK, LTRIM, or overwrites that reduce memory may still be allowed depending on command flags and behavior.

So your mental model is basically correct:

text
used_memory < maxmemory
  -> Redis may accept a write
  -> that write may push memory above maxmemory

used_memory > maxmemory + noeviction
  -> Redis rejects further memory-growing writes with OOM
  -> reads and some cleanup commands still work

The error is usually:

text
OOM command not allowed when used memory > 'maxmemory'

The recovery is to delete/expire data, increase maxmemory, or change the eviction policy if Redis is acting as a cache rather than a durable data store.