Use madvise(MADV_DONTNEED) to release memory to reduce COW (#8974)
## 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 m...
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