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In some cases, users will abuse lua eval. Each EVAL call generates
a new lua script, which is added to the lua interpreter and cached
to redis-server, consuming a large amount of memory over time.
Since EVAL is mostly the one that abuses the lua cache, and these
won't have pipeline issues (i.e. the script won't disappear
unexpectedly,
and cause errors like it would with SCRIPT LOAD and EVALSHA),
we implement a plain FIFO LRU eviction only for these (not for
scripts loaded with SCRIPT LOAD).
### Implementation notes:
When not abused we'll probably have less than 100 scripts, and when
abused we'll have many thousands. So we use a hard coded value of 500
scripts. And considering that we don't have many scripts, then unlike
keys, we don't need to worry about the memory usage of keeping a true
sorted LRU linked list. We compute the SHA of each script anyway,
and put the script in a dict, we can store a listNode there, and use
it for quick removal and re-insertion into an LRU list each time the
script is used.
### New interfaces:
At the same time, a new `evicted_scripts` field is added to
INFO, which represents the number of evicted eval scripts. Users
can check it to see if they are abusing EVAL.
### benchmark:
`./src/redis-benchmark -P 10 -n 1000000 -r 10000000000 eval "return
__rand_int__" 0`
The simple abuse of eval benchmark test that will create 1 million EVAL
scripts. The performance has been improved by 50%, and the max latency
has dropped from 500ms to 13ms (this may be caused by table expansion
inside Lua when the number of scripts is large). And in the INFO memory,
it used to consume 120MB (server cache) + 310MB (lua engine), but now
it only consumes 70KB (server cache) + 210KB (lua_engine) because of
the scripts eviction.
For non-abusive case of about 100 EVAL scripts, there's no noticeable
change in performance or memory usage.
### unlikely potentially breaking change:
in theory, a user can maybe load a
script with EVAL and then use EVALSHA to call it (by calculating the
SHA1 value on the client side), it could be that if we read the docs
carefully we'll realized it's a valid scenario, but we suppose it's
extremely rare. So it may happen that EVALSHA acts on a script created
by EVAL, and the script is evicted and EVALSHA returns a NOSCRIPT error.
that is if you have more than 500 scripts being used in the same
transaction / pipeline.
This solves the second point in #13102.
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