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## Backgroud As we know, after `fork`, one process will copy pages when writing data to these pages(CoW), and another process still keep old pages, they totally cost more memory. For redis, we suffered that redis consumed much memory when the fork child is serializing key/values, even that maybe cause OOM. But actually we find, in redis fork child process, the child process don't need to keep some memory and parent process may write or update that, for example, child process will never access the key-value that is serialized but users may update it in parent process. So we think it may reduce COW if the child process release memory that it is not needed. ## Implementation For releasing key value in child process, we may think we call `decrRefCount` to free memory, but i find the fork child process still use much memory when we don't write any data to redis, and it costs much more time that slows down bgsave. Maybe because memory allocator doesn't really release memory to OS, and it may modify some inner data for this free operation, especially when we free small objects. Moreover, CoW is based on pages, so it is a easy way that we only free the memory bulk that is not less than kernel page size. madvise(MADV_DONTNEED) can quickly release specified region pages to OS bypassing memory allocator, and allocator still consider that this memory still is used and don't change its inner data. There are some buffers we can release in the fork child process: - **Serialized key-values** the fork child process never access serialized key-values, so we try to free them. Because we only can release big bulk memory, and it is time consumed to iterate all items/members/fields/entries of complex data type. So we decide to iterate them and try to release them only when their average size of item/member/field/entry is more than page size of OS. - **Replication backlog** Because replication backlog is a cycle buffer, it will be changed quickly if redis has heavy write traffic, but in fork child process, we don't need to access that. - **Client buffers** If clients have requests during having the fork child process, clients' buffer also be changed frequently. The memory includes client query buffer, output buffer, and client struct used memory. To get child process peak private dirty memory, we need to count peak memory instead of last used memory, because the child process may continue to release memory (since COW used to only grow till now, the last was equivalent to the peak). Also we're adding a new `current_cow_peak` info variable (to complement the existing `current_cow_size`) Co-authored-by: Oran Agra <oran@redislabs.com>
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. |
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. |
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). |
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 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.
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