- 25 Oct, 2021 1 commit
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Wang Yuan authored
## Background For redis master, one replica uses one copy of replication buffer, that is a big waste of memory, more replicas more waste, and allocate/free memory for every reply list also cost much. If we set client-output-buffer-limit small and write traffic is heavy, master may disconnect with replicas and can't finish synchronization with replica. If we set client-output-buffer-limit big, master may be OOM when there are many replicas that separately keep much memory. Because replication buffers of different replica client are the same, one simple idea is that all replicas only use one replication buffer, that will effectively save memory. Since replication backlog content is the same as replicas' output buffer, now we can discard replication backlog memory and use global shared replication buffer to implement replication backlog mechanism. ## Implementation I create one global "replication buffer" which contains content of replication stream. The structure of "replication buffer" is similar to the reply list that exists in every client. But the node of list is `replBufBlock`, which has `id, repl_offset, refcount` fields. ```c /* Replication buffer blocks is the list of replBufBlock. * * +--------------+ +--------------+ +--------------+ * | refcount = 1 | ... | refcount = 0 | ... | refcount = 2 | * +--------------+ +--------------+ +--------------+ * | / \ * | / \ * | / \ * Repl Backlog Replia_A Replia_B * * Each replica or replication backlog increments only the refcount of the * 'ref_repl_buf_node' which it points to. So when replica walks to the next * node, it should first increase the next node's refcount, and when we trim * the replication buffer nodes, we remove node always from the head node which * refcount is 0. If the refcount of the head node is not 0, we must stop * trimming and never iterate the next node. */ /* Similar with 'clientReplyBlock', it is used for shared buffers between * all replica clients and replication backlog. */ typedef struct replBufBlock { int refcount; /* Number of replicas or repl backlog using. */ long long id; /* The unique incremental number. */ long long repl_offset; /* Start replication offset of the block. */ size_t size, used; char buf[]; } replBufBlock; ``` So now when we feed replication stream into replication backlog and all replicas, we only need to feed stream into replication buffer `feedReplicationBuffer`. In this function, we set some fields of replication backlog and replicas to references of the global replication buffer blocks. And we also need to check replicas' output buffer limit to free if exceeding `client-output-buffer-limit`, and trim replication backlog if exceeding `repl-backlog-size`. When sending reply to replicas, we also need to iterate replication buffer blocks and send its content, when totally sending one block for replica, we decrease current node count and increase the next current node count, and then free the block which reference is 0 from the head of replication buffer blocks. Since now we use linked list to manage replication backlog, it may cost much time for iterating all linked list nodes to find corresponding replication buffer node. So we create a rax tree to store some nodes for index, but to avoid rax tree occupying too much memory, i record one per 64 nodes for index. Currently, to make partial resynchronization as possible as much, we always let replication backlog as the last reference of replication buffer blocks, backlog size may exceeds our setting if slow replicas that reference vast replication buffer blocks, and this method doesn't increase memory usage since they share replication buffer. To avoid freezing server for freeing unreferenced replication buffer blocks when we need to trim backlog for exceeding backlog size setting, we trim backlog incrementally (free 64 blocks per call now), and make it faster in `beforeSleep` (free 640 blocks). ### Other changes - `mem_total_replication_buffers`: we add this field in INFO command, it means the total memory of replication buffers used. - `mem_clients_slaves`: now even replica is slow to replicate, and its output buffer memory is not 0, but it still may be 0, since replication backlog and replicas share one global replication buffer, only if replication buffer memory is more than the repl backlog setting size, we consider the excess as replicas' memory. Otherwise, we think replication buffer memory is the consumption of repl backlog. - Key eviction Since all replicas and replication backlog share global replication buffer, we think only the part of exceeding backlog size the extra separate consumption of replicas. Because we trim backlog incrementally in the background, backlog size may exceeds our setting if slow replicas that reference vast replication buffer blocks disconnect. To avoid massive eviction loop, we don't count the delayed freed replication backlog into used memory even if there are no replicas, i.e. we also regard this memory as replicas's memory. - `client-output-buffer-limit` check for replica clients It doesn't make sense to set the replica clients output buffer limit lower than the repl-backlog-size config (partial sync will succeed and then replica will get disconnected). Such a configuration is ignored (the size of repl-backlog-size will be used). This doesn't have memory consumption implications since the replica client will share the backlog buffers memory. - Drop replication backlog after loading data if needed We always create replication backlog if server is a master, we need it because we put DELs in it when loading expired keys in RDB, but if RDB doesn't have replication info or there is no rdb, it is not possible to support partial resynchronization, to avoid extra memory of replication backlog, we drop it. - Multi IO threads Since all replicas and replication backlog use global replication buffer, if I/O threads are enabled, to guarantee data accessing thread safe, we must let main thread handle sending the output buffer to all replicas. But before, other IO threads could handle sending output buffer of all replicas. ## Other optimizations This solution resolve some other problem: - When replicas disconnect with master since of out of output buffer limit, releasing the output buffer of replicas may freeze server if we set big `client-output-buffer-limit` for replicas, but now, it doesn't cause freezing. - This implementation may mitigate reply list copy cost time(also freezes server) when one replication has huge reply buffer and another replica can copy buffer for full synchronization. now, we just copy reference info, it is very light. - If we set replication backlog size big, it also may cost much time to copy replication backlog into replica's output buffer. But this commit eliminates this problem. - Resizing replication backlog size doesn't empty current replication backlog content.
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- 06 Sep, 2020 1 commit
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Oran Agra authored
this is important when running a test with --loop
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- 06 Aug, 2020 1 commit
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Oran Agra authored
Diskless master has some inherent latencies. 1) fork starts with delay from cron rather than immediately 2) replica is put online only after an ACK. but the ACK was sent only once a second. 3) but even if it would arrive immediately, it will not register in case cron didn't yet detect that the fork is done. Besides that, when a replica disconnects, it doesn't immediately attempts to re-connect, it waits for replication cron (one per second). in case it was already online, it may be important to try to re-connect as soon as possible, so that the backlog at the master doesn't vanish. In case it disconnected during rdb transfer, one can argue that it's not very important to re-connect immediately, but this is needed for the "diskless loading short read" test to be able to run 100 iterations in 5 seconds, rather than 3 (waiting for replication cron re-connection) changes in this commit: 1) sync command starts a fork immediately if no sync_delay is configured 2) replica sends REPLCONF ACK when done reading the rdb (rather than on 1s cron) 3) when a replica unexpectedly disconnets, it immediately tries to re-connect rather than waiting 1s 4) when when a child exits, if there is another replica waiting, we spawn a new one right away, instead of waiting for 1s replicationCron. 5) added a call to connectWithMaster from replicationSetMaster. which is called from the REPLICAOF command but also in 3 places in cluster.c, in all of these the connection attempt will now be immediate instead of delayed by 1 second. side note: we can add a call to rdbPipeReadHandler in replconfCommand when getting a REPLCONF ACK from the replica to solve a race where the replica got the entire rdb and EOF marker before we detected that the pipe was closed. in the test i did see this race happens in one about of some 300 runs, but i concluded that this race is unlikely in real life (where the replica is on another host and we're more likely to first detect the pipe was closed. the test runs 100 iterations in 3 seconds, so in some cases it'll take 4 seconds instead (waiting for another REPLCONF ACK). Removing unneeded startBgsaveForReplication from updateSlavesWaitingForBgsave Now that CheckChildrenDone is calling the new replicationStartPendingFork (extracted from serverCron) there's actually no need to call startBgsaveForReplication from updateSlavesWaitingForBgsave anymore, since as soon as updateSlavesWaitingForBgsave returns, CheckChildrenDone is calling replicationStartPendingFork that handles that anyway. The code in updateSlavesWaitingForBgsave had a bug in which it ignored repl-diskless-sync-delay, but removing that code shows that this bug was hiding another bug, which is that the max_idle should have used >= and not >, this one second delay has a big impact on my new test.
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- 28 Jul, 2020 1 commit
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Oran Agra authored
- the test now waits for specific set of log messages rather than wait for timeout looking for just one message. - we don't wanna sample the current length of the log after an action, due to a race, we need to start the search from the line number of the last message we where waiting for. - when attempting to trigger a full sync, use multi-exec to avoid a race where the replica manages to re-connect before we completed the set of actions that should force a full sync. - fix verify_log_message which was broken and unused
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- 10 Jul, 2020 1 commit
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Oran Agra authored
tests were sensitive to additional log lines appearing in the log causing the search to come empty handed. instead of just looking for the n last log lines, capture the log lines before performing the action, and then search from that offset.
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- 24 Oct, 2019 1 commit
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Oran Agra authored
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- 30 Jul, 2019 1 commit
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Oran Agra authored
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- 22 Jul, 2019 1 commit
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Oran Agra authored
Other changes: * fix memory leak in error handling of rdb loading of type OBJ_MODULE
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