b26e8e3213
ZRANGE BYSCORE/BYLEX with [LIMIT offset count] option was
using every level in skiplist to jump to the first/last node in range,
but only use level[0] in skiplist to locate the node at offset, resulting
in sub-optimal performance using LIMIT:
```
while (ln && offset--) {
if (reverse) {
ln = ln->backward;
} else {
ln = ln->level[0].forward;
}
}
```
It could be slow when offset is very big. We can get the total rank of
the offset location and use skiplist to jump to it. It is an improvement
from O(offset) to O(log rank).
Below shows how this is implemented (if the offset is positve):
Use the skiplist to seach for the first element in the range, record its
rank `rank_0`, so we can have the rank of the target node `rank_t`.
Meanwhile we record the last node we visited which has zsl->level-1
levels and its rank `rank_1`. Then we start from the zsl->level-1 node,
use skiplist to go forward `rank_t-rank_1` nodes to reach the target node.
It is very similiar when the offset is reversed.
Note that if `rank_t` is very close to `rank_0`, we just start from the first
element in range and go node by node, this for the case when zsl->level-1
node is to far away and it is quicker to reach the target node by node.
Here is a test using a random generated zset including 10000 elements
(with different positive scores), doing a bench mark which compares how
fast the `ZRANGE` command is exucuted before and after the optimization.
The start score is set to 0 and the count is set to 1 to make sure that
most of the time is spent on locating the offset.
```
memtier_benchmark -h 127.0.0.1 -p 6379 --command="zrange test 0 +inf byscore limit <offset> 1"
```
| offset | QPS(unstable) | QPS(optimized) |
|--------|--------|--------|
| 10 | 73386.02 | 74819.82 |
| 1000 | 48084.96 | 73177.73 |
| 2000 | 31156.79 | 72805.83 |
| 5000 | 10954.83 | 71218.21 |
With the result above, we can see that the original code is greatly
slowed down when offset gets bigger, and with the optimization the
speed is almost not affected.
Similiar results are generated when testing reversed offset:
```
memtier_benchmark -h 127.0.0.1 -p 6379 --command="zrange test +inf 0 byscore rev limit <offset> 1"
```
| offset | QPS(unstable) | QPS(optimized) |
|--------|--------|--------|
| 10 | 74505.14 | 71653.67 |
| 1000 | 46829.25 | 72842.75 |
| 2000 | 28985.48 | 73669.01 |
| 5000 | 11066.22 | 73963.45 |
And the same conclusion is drawn from the tests of ZRANGE BYLEX.
Redis Test Suite
The normal execution mode of the test suite involves starting and manipulating
local redis-server instances, inspecting process state, log files, etc.
The test suite also supports execution against an external server, which is
enabled using the --host and --port parameters. When executing against an
external server, tests tagged external:skip are skipped.
There are additional runtime options that can further adjust the test suite to match different external server configurations:
| Option | Impact |
|---|---|
--singledb |
Only use database 0, don't assume others are supported. |
--ignore-encoding |
Skip all checks for specific encoding. |
--ignore-digest |
Skip key value digest validations. |
--cluster-mode |
Run in strict Redis Cluster compatibility mode. |
--large-memory |
Enables tests that consume more than 100mb |
Tags
Tags are applied to tests to classify them according to the subsystem they test, but also to indicate compatibility with different run modes and required capabilities.
Tags can be applied in different context levels:
start_servercontexttagscontext that bundles several tests together- A single test context.
The following compatibility and capability tags are currently used:
| Tag | Indicates |
|---|---|
external:skip |
Not compatible with external servers. |
cluster:skip |
Not compatible with --cluster-mode. |
large-memory |
Test that requires more than 100mb |
tls:skip |
Not compatible with --tls. |
needs:repl |
Uses replication and needs to be able to SYNC from server. |
needs:debug |
Uses the DEBUG command or other debugging focused commands (like OBJECT REFCOUNT). |
needs:pfdebug |
Uses the PFDEBUG command. |
needs:config-maxmemory |
Uses CONFIG SET to manipulate memory limit, eviction policies, etc. |
needs:config-resetstat |
Uses CONFIG RESETSTAT to reset statistics. |
needs:reset |
Uses RESET to reset client connections. |
needs:save |
Uses SAVE or BGSAVE to create an RDB file. |
When using an external server (--host and --port), filtering using the
external:skip tags is done automatically.
When using --cluster-mode, filtering using the cluster:skip tag is done
automatically.
When not using --large-memory, filtering using the largemem:skip tag is done
automatically.
In addition, it is possible to specify additional configuration. For example, to
run tests on a server that does not permit SYNC use:
./runtest --host <host> --port <port> --tags -needs:repl