server: set multiple concurrentReadTx instances share one txReadBuffer. Approach #2

[wilsonwang371]

This is a different approach for the similar issue trying to resolve in #12933

Currently, the concurrentReadTx does a copy of the txReadBuffer inside backend readTx. However, this buffer is never modified by concurrentReadTx. Therefore, we can just use a single copy of the txReadBuffer as long as there are no new changes committed to backend DB.

Therefore, we use a cached txReadBuffer to avoid excessive deep copy operations for the new read transaction.

[wilsonwang371]

This is a patch inspired by #12529

[codecov-commenter]

Codecov Report

Merging #12935 (d422795) into master (aeb9b5f) will decrease coverage by 0.34%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master   #12935      +/-   ##
==========================================
- Coverage   73.31%   72.97%   -0.35%     
==========================================
  Files         430      430              
  Lines       34182    34676     +494     
==========================================
+ Hits        25060    25304     +244     
- Misses       7194     7393     +199     
- Partials     1928     1979      +51     

Flag	Coverage Δ
all	72.97% <83.33%> (-0.35%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/mvcc/backend/backend.go	80.07% <75.00%> (-0.32%)	⬇️
server/mvcc/backend/tx_buffer.go	92.50% <100.00%> (+0.39%)	⬆️
client/v3/txn.go	68.42% <0.00%> (-31.58%)	⬇️
server/etcdserver/api/rafthttp/peer.go	77.41% <0.00%> (-18.07%)	⬇️
server/etcdserver/api/rafthttp/peer_status.go	89.28% <0.00%> (-10.72%)	⬇️
client/pkg/v3/tlsutil/tlsutil.go	83.33% <0.00%> (-8.34%)	⬇️
server/config/config.go	66.00% <0.00%> (-8.25%)	⬇️
client/v3/leasing/cache.go	87.50% <0.00%> (-4.61%)	⬇️
server/etcdserver/api/v3rpc/member.go	92.45% <0.00%> (-3.78%)	⬇️
server/etcdserver/api/rafthttp/msgappv2_codec.go	67.88% <0.00%> (-3.67%)	⬇️
... and 22 more

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[wilsonwang371]

I am going to run a performance evaluation of this patch later.

[wilsonwang371]

@gyuho @ptabor @jingyih

Need u guys to take a look at this patch too. This patch can significantly increase the read qps performance.

[wilsonwang371]

Just realized this patch is shorter than #12933, but the performance will be most probably lower than #12933.

The reason for this is that write requests will have to do a copy of the readTx txReadBuffer each time doing a commit.

But I will still upload a performance evaluation for this patch too.

[wilsonwang371]

Never mind about this patch. Its initial performance is horrible.