zhixinwen opened a new pull request, #3061:
URL: https://github.com/apache/kvrocks/pull/3061
Add ack mechanism in WAIT and replication logic.
When a replica sync delta change from master, it would send a `replconf ack
<sequence number>" to master.`FeedSlaveThread::staticReadCallback` will record
the ack and unblock WAIT command.
I wrote a simple loadtest script to test the setup with one master + slave
setup locally. The test has 120 concurrent workers, each calling SET with 100
bytes value:
```
Test Configuration:
Type: set
Duration: 1m0.000947872s
Concurrency: 120
Value Size: 100 bytes
Overall Statistics:
Total Requests: 4303221
Successful: 4303101 (100.00%)
Failed: 120 (0.00%)
Throughput: 71719.22 requests/second
```
If after each SET, we immediately follow a WAIT:
```
Test Configuration:
Type: set-wait
Duration: 1m0.000804367s
Concurrency: 120
Value Size: 100 bytes
Wait Replicas: 1
Overall Statistics:
Total Requests: 2237526
Successful: 2237406 (99.99%)
Failed: 120 (0.01%)
Throughput: 37291.60 requests/second
```
When I first implement this feature, I tried to be smart and did the
following:
1. When there is no client waiting, replica ACK every 1s.
2. When there is client waiting, in `incrementBatchLoopCB`, master would
send `_getack`.
3. When replica receives `_getack` , it would ack immediately.
This logic did not work well, I guess the issue is a client can send out too
many `_getack` when it is blocked. And replica is busy with processing
`_getack` instead of processing data, which makes the replication processing a
lot slower:
```
Test Configuration:
Type: set-wait
Duration: 2m0.002154771s
Concurrency: 120
Value Size: 1024 bytes
Wait Replicas: 1
Overall Statistics:
Total Requests: 22996
Successful: 21916 (95.30%)
Failed: 1080 (4.70%)
Throughput: 191.63 requests/second
```
I want to see if this mechanism is good enough for maintainers and I am open
to suggestions to make it more efficient.
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