reconciler: Reuse incremental round state

[joamaki]

The 'results' map was getting allocated fresh for each round and with large batch size the cpu profile showed bunch of time being spent on rehashing.

Benchmark before:

1000000 objects reconciled in 1.44 seconds (batch size 1000) Throughput 694132.66 objects per second
1135MB total allocated, 6014828 in-use objects, 338MB bytes in use 2.18user 0.22system 0:01.53elapsed 157%CPU (0avgtext+0avgdata 678224maxresident)k 0inputs+0outputs (0major+208598minor)pagefaults 0swaps

Benchmark after:

1000000 objects reconciled in 1.44 seconds (batch size 1000) Throughput 696145.36 objects per second
886MB total allocated, 6014843 in-use objects, 338MB bytes in use 1.90user 0.19system 0:01.53elapsed 136%CPU (0avgtext+0avgdata 662688maxresident)k 0inputs+0outputs (0major+211455minor)pagefaults 0swaps

[github-actions]

$ make
go build ./...
go: downloading go1.24.0 (linux/amd64)
go: downloading go.yaml.in/yaml/v3 v3.0.3
go: downloading github.com/cilium/hive v0.0.0-20250731144630-28e7a35ed227
go: downloading golang.org/x/time v0.5.0
go: downloading github.com/spf13/cobra v1.8.0
go: downloading github.com/spf13/pflag v1.0.5
go: downloading github.com/cilium/stream v0.0.0-20240209152734-a0792b51812d
go: downloading github.com/liggitt/tabwriter v0.0.0-20181228230101-89fcab3d43de
go: downloading github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc
go: downloading github.com/mitchellh/mapstructure v1.5.0
go: downloading go.uber.org/dig v1.17.1
go: downloading golang.org/x/term v0.16.0
go: downloading github.com/spf13/viper v1.18.2
go: downloading golang.org/x/sys v0.17.0
go: downloading golang.org/x/tools v0.17.0
go: downloading github.com/spf13/cast v1.6.0
go: downloading github.com/fsnotify/fsnotify v1.7.0
go: downloading github.com/sagikazarmark/slog-shim v0.1.0
go: downloading github.com/spf13/afero v1.11.0
go: downloading github.com/subosito/gotenv v1.6.0
go: downloading github.com/hashicorp/hcl v1.0.0
go: downloading gopkg.in/ini.v1 v1.67.0
go: downloading github.com/magiconair/properties v1.8.7
go: downloading github.com/pelletier/go-toml/v2 v2.1.0
go: downloading gopkg.in/yaml.v3 v3.0.1
go: downloading golang.org/x/text v0.14.0
go test ./... -cover -vet=all -test.count 1
go: downloading github.com/stretchr/testify v1.8.4
go: downloading go.uber.org/goleak v1.3.0
go: downloading golang.org/x/exp v0.0.0-20240119083558-1b970713d09a
go: downloading github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2
ok  	github.com/cilium/statedb	21.775s	coverage: 80.8% of statements
ok  	github.com/cilium/statedb/index	0.014s	coverage: 28.7% of statements
ok  	github.com/cilium/statedb/internal	0.027s	coverage: 46.7% of statements
ok  	github.com/cilium/statedb/part	4.326s	coverage: 88.3% of statements
ok  	github.com/cilium/statedb/reconciler	0.245s	coverage: 89.1% of statements
	github.com/cilium/statedb/reconciler/benchmark		coverage: 0.0% of statements
	github.com/cilium/statedb/reconciler/example		coverage: 0.0% of statements
go test -race ./... -test.count 1
ok  	github.com/cilium/statedb	108.319s
ok  	github.com/cilium/statedb/index	1.013s
ok  	github.com/cilium/statedb/internal	1.034s
ok  	github.com/cilium/statedb/part	32.641s
ok  	github.com/cilium/statedb/reconciler	1.358s
?   	github.com/cilium/statedb/reconciler/benchmark	[no test files]
?   	github.com/cilium/statedb/reconciler/example	[no test files]
go test ./... -bench . -benchmem -test.run xxx
goos: linux
goarch: amd64
pkg: github.com/cilium/statedb
cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
BenchmarkDB_WriteTxn_1-4                    	  542863	      1971 ns/op	    507281 objects/sec	    1288 B/op	      24 allocs/op
BenchmarkDB_WriteTxn_10-4                   	 1379109	       898.3 ns/op	   1113220 objects/sec	     677 B/op	       9 allocs/op
BenchmarkDB_WriteTxn_100-4                  	 1892676	       698.3 ns/op	   1432134 objects/sec	     419 B/op	       7 allocs/op
BenchmarkDB_WriteTxn_1000-4                 	 1844871	       663.3 ns/op	   1507550 objects/sec	     363 B/op	       7 allocs/op
BenchmarkDB_WriteTxn_100_SecondaryIndex-4   	  585412	      1922 ns/op	    520409 objects/sec	    1229 B/op	      37 allocs/op
BenchmarkDB_NewWriteTxn-4                   	 1500297	       800.5 ns/op	     544 B/op	       8 allocs/op
BenchmarkDB_NewReadTxn-4                    	769796583	         1.610 ns/op	       0 B/op	       0 allocs/op
BenchmarkDB_Modify-4                        	    1735	    746467 ns/op	   1339645 objects/sec	  421796 B/op	    8130 allocs/op
BenchmarkDB_GetInsert-4                     	    1618	    815095 ns/op	   1226852 objects/sec	  405756 B/op	    8129 allocs/op
BenchmarkDB_RandomInsert-4                  	    1864	    710333 ns/op	   1407791 objects/sec	  397264 B/op	    7129 allocs/op
BenchmarkDB_RandomReplace-4                 	     396	   3066885 ns/op	    326064 objects/sec	 1724165 B/op	   46155 allocs/op
BenchmarkDB_SequentialInsert-4              	    2025	    680661 ns/op	   1469159 objects/sec	  363417 B/op	    7101 allocs/op
BenchmarkDB_SequentialInsert_Prefix-4       	     441	   2808982 ns/op	    356001 objects/sec	 3475218 B/op	   56557 allocs/op
BenchmarkDB_Changes_Baseline-4              	    1453	    887339 ns/op	   1126965 objects/sec	  436305 B/op	   10193 allocs/op
BenchmarkDB_Changes-4                       	     818	   1610889 ns/op	    620775 objects/sec	  760631 B/op	   14377 allocs/op
BenchmarkDB_RandomLookup-4                  	   23898	     49711 ns/op	  20116416 objects/sec	       0 B/op	       0 allocs/op
BenchmarkDB_SequentialLookup-4              	   31075	     38535 ns/op	  25950220 objects/sec	       0 B/op	       0 allocs/op
BenchmarkDB_Prefix_SecondaryIndex-4         	    7467	    144706 ns/op	   6910575 objects/sec	  125114 B/op	    1031 allocs/op
BenchmarkDB_FullIteration_All-4             	    1258	    989746 ns/op	 101036003 objects/sec	     320 B/op	      11 allocs/op
BenchmarkDB_FullIteration_Get-4             	     229	   5209598 ns/op	  19195339 objects/sec	       0 B/op	       0 allocs/op
BenchmarkDB_FullIteration_ReadTxnGet-4      	     246	   4850827 ns/op	  20615043 objects/sec	       0 B/op	       0 allocs/op
BenchmarkDB_PropagationDelay-4              	  623749	      2108 ns/op	        17.00 50th_µs	        21.00 90th_µs	       130.0 99th_µs	    1440 B/op	      22 allocs/op
BenchmarkWatchSet_4-4                       	 2140526	       560.6 ns/op	     320 B/op	       5 allocs/op
BenchmarkWatchSet_16-4                      	  608238	      1695 ns/op	    1096 B/op	       5 allocs/op
BenchmarkWatchSet_128-4                     	   84067	     14542 ns/op	    8904 B/op	       5 allocs/op
BenchmarkWatchSet_1024-4                    	    8748	    140149 ns/op	   73744 B/op	       5 allocs/op
PASS
ok  	github.com/cilium/statedb	34.561s
PASS
ok  	github.com/cilium/statedb/index	0.003s
PASS
ok  	github.com/cilium/statedb/internal	0.003s
goos: linux
goarch: amd64
pkg: github.com/cilium/statedb/part
cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
Benchmark_Uint64Map_Random-4                  	    1418	    801776 ns/op	   1247231 items/sec	 2573701 B/op	    7060 allocs/op
Benchmark_Uint64Map_Sequential-4              	    1800	    692248 ns/op	   1444570 items/sec	 2266257 B/op	    6755 allocs/op
Benchmark_Uint64Map_Sequential_Insert-4       	    1749	    636507 ns/op	   1571075 items/sec	 2258257 B/op	    5754 allocs/op
Benchmark_Uint64Map_Sequential_Txn_Insert-4   	   10000	    108007 ns/op	   9258665 items/sec	   89068 B/op	    2035 allocs/op
Benchmark_Uint64Map_Random_Insert-4           	    1653	    729423 ns/op	   1370947 items/sec	 2563316 B/op	    6042 allocs/op
Benchmark_Uint64Map_Random_Txn_Insert-4       	    6634	    186149 ns/op	   5372051 items/sec	  139551 B/op	    2597 allocs/op
Benchmark_Insert_RootOnlyWatch-4              	   12235	     98141 ns/op	  10189456 objects/sec	   74270 B/op	    2041 allocs/op
Benchmark_Insert-4                            	    8445	    146195 ns/op	   6840171 objects/sec	  190386 B/op	    3074 allocs/op
Benchmark_Modify-4                            	   14593	     86764 ns/op	  11525575 objects/sec	   71007 B/op	    1017 allocs/op
Benchmark_GetInsert-4                         	   10000	    107967 ns/op	   9262096 objects/sec	   71018 B/op	    1017 allocs/op
Benchmark_Replace-4                           	42608322	        27.74 ns/op	  36045410 objects/sec	       0 B/op	       0 allocs/op
Benchmark_Replace_RootOnlyWatch-4             	43170369	        27.81 ns/op	  35953818 objects/sec	       0 B/op	       0 allocs/op
Benchmark_txn_1-4                             	 5378364	       220.4 ns/op	   4537687 objects/sec	     216 B/op	       4 allocs/op
Benchmark_txn_10-4                            	10805846	       110.9 ns/op	   9017644 objects/sec	      90 B/op	       2 allocs/op
Benchmark_txn_100-4                           	13110589	        91.44 ns/op	  10936672 objects/sec	      89 B/op	       2 allocs/op
Benchmark_txn_1000-4                          	11086810	       102.0 ns/op	   9803973 objects/sec	      77 B/op	       2 allocs/op
Benchmark_txn_delete_1-4                      	 1930861	       659.6 ns/op	   1516058 objects/sec	    2952 B/op	       5 allocs/op
Benchmark_txn_delete_10-4                     	 8956024	       135.5 ns/op	   7379679 objects/sec	     336 B/op	       1 allocs/op
Benchmark_txn_delete_100-4                    	15085370	        81.60 ns/op	  12254626 objects/sec	      79 B/op	       1 allocs/op
Benchmark_txn_delete_1000-4                   	16598912	        72.39 ns/op	  13813525 objects/sec	      27 B/op	       1 allocs/op
Benchmark_Get-4                               	   49908	     23762 ns/op	  42083418 objects/sec	       0 B/op	       0 allocs/op
Benchmark_Iterate-4                           	  189880	      6249 ns/op	 160037136 objects/sec	     104 B/op	       4 allocs/op
Benchmark_Hashmap_Insert-4                    	   19586	     60611 ns/op	  16498676 objects/sec	   74265 B/op	      20 allocs/op
Benchmark_Hashmap_Get_Uint64-4                	  175610	      6835 ns/op	 146309151 objects/sec	       0 B/op	       0 allocs/op
Benchmark_Hashmap_Get_Bytes-4                 	  139180	      8612 ns/op	 116122521 objects/sec	       0 B/op	       0 allocs/op
Benchmark_Delete_Random-4                     	      16	  70730136 ns/op	   1413825 objects/sec	71860455 B/op	  137797 allocs/op
PASS
ok  	github.com/cilium/statedb/part	32.657s
PASS
ok  	github.com/cilium/statedb/reconciler	0.004s
?   	github.com/cilium/statedb/reconciler/benchmark	[no test files]
?   	github.com/cilium/statedb/reconciler/example	[no test files]
go run ./reconciler/benchmark -quiet
1000000 objects reconciled in 2.20 seconds (batch size 1000)
Throughput 454117.43 objects per second
885MB total allocated, 6014798 in-use objects, 338MB bytes in use