X zodb/cache: Also free OCE entries on GC

Until now we were only GC'ing RCE entries, but OCE entries (top-level cache entry for an OID) were never GC'ed thus allowing cache size to grow infinitely. Even though the average N(allocations) stays the same this change lowers pressure on amount of memory allocated and thus helping avoid some GC speeds things up (see attachment). Notes: 1. Hit*/size=0 increase is probably due to Cache.entryMap going to empty/1-element back and forth constantly. size/0 is not going to happen in practice and so this wat we can accept regression here. ( still NoHit*/size=0 works faster ). 2. HitProc/size=* "regression should be within noise" name old time/op new time/op delta NoopStorage-4 56.6ns ± 1% 56.2ns ± 0% ~ (p=0.079 n=5+5) CacheStartup-4 1.24µs ± 6% 1.28µs ± 9% ~ (p=0.595 n=5+5) CacheNoHit/size=0-4 1.44µs ± 4% 0.93µs ± 2% -35.26% (p=0.008 n=5+5) CacheNoHit/size=16-4 1.43µs ± 5% 0.92µs ± 1% -35.19% (p=0.008 n=5+5) CacheNoHit/size=128-4 1.45µs ± 3% 0.94µs ± 1% -35.03% (p=0.008 n=5+5) CacheNoHit/size=512-4 1.44µs ± 4% 0.97µs ± 2% -32.64% (p=0.008 n=5+5) CacheNoHit/size=4096-4 1.45µs ± 2% 1.07µs ± 0% -25.85% (p=0.008 n=5+5) CacheHit/size=0-4 131ns ± 2% 276ns ±22% +110.99% (p=0.008 n=5+5) CacheHit/size=16-4 122ns ± 1% 121ns ± 1% ~ (p=0.079 n=5+5) CacheHit/size=128-4 126ns ± 2% 125ns ± 1% ~ (p=0.563 n=5+5) CacheHit/size=512-4 127ns ± 1% 126ns ± 0% ~ (p=0.095 n=5+4) CacheHit/size=4096-4 128ns ± 0% 128ns ± 0% ~ (p=0.556 n=5+4) NoopStoragePar-4 30.6ns ± 4% 31.2ns ±10% ~ (p=0.690 n=5+5) CacheStartupPar-4 1.44µs ± 5% 1.43µs ± 3% ~ (p=0.690 n=5+5) CacheNoHitPar/size=0-4 1.62µs ± 4% 1.04µs ± 1% -35.76% (p=0.008 n=5+5) CacheNoHitPar/size=16-4 1.65µs ± 4% 1.05µs ± 1% -36.39% (p=0.008 n=5+5) CacheNoHitPar/size=128-4 1.64µs ± 5% 1.05µs ± 1% -35.84% (p=0.008 n=5+5) CacheNoHitPar/size=512-4 1.62µs ± 3% 1.08µs ± 1% -33.10% (p=0.008 n=5+5) CacheNoHitPar/size=4096-4 1.68µs ± 1% 1.18µs ± 0% -29.44% (p=0.008 n=5+5) CacheHitPar/size=0-4 215ns ± 0% 383ns ± 2% +78.23% (p=0.008 n=5+5) CacheHitPar/size=16-4 214ns ± 2% 214ns ± 0% ~ (p=0.786 n=5+5) CacheHitPar/size=128-4 210ns ± 0% 209ns ± 0% ~ (p=0.079 n=5+5) CacheHitPar/size=512-4 207ns ± 0% 206ns ± 0% -0.48% (p=0.008 n=5+5) CacheHitPar/size=4096-4 204ns ± 0% 202ns ± 0% -0.98% (p=0.000 n=5+4) NoopStorageProc-4 31.4ns ± 7% 33.7ns ± 5% ~ (p=0.151 n=5+5) CacheStartupProc-4 1.13µs ± 5% 1.12µs ± 3% ~ (p=0.690 n=5+5) CacheNoHitProc/size=0-4 1.12µs ± 5% 0.62µs ± 1% -44.52% (p=0.008 n=5+5) CacheNoHitProc/size=16-4 1.14µs ± 6% 0.63µs ± 1% -45.14% (p=0.008 n=5+5) CacheNoHitProc/size=128-4 1.06µs ± 5% 0.64µs ± 2% -40.12% (p=0.008 n=5+5) CacheNoHitProc/size=512-4 1.14µs ±11% 0.69µs ± 4% -39.87% (p=0.008 n=5+5) CacheNoHitProc/size=4096-4 1.14µs ± 9% 0.68µs ± 2% -40.21% (p=0.008 n=5+5) CacheHitProc/size=0-4 56.5ns ± 7% 84.6ns ±14% +49.66% (p=0.008 n=5+5) CacheHitProc/size=16-4 55.8ns ± 0% 62.0ns ± 6% +11.03% (p=0.008 n=5+5) CacheHitProc/size=128-4 56.6ns ± 0% 60.9ns ± 4% +7.63% (p=0.008 n=5+5) CacheHitProc/size=512-4 57.3ns ± 0% 64.1ns ± 7% +11.83% (p=0.016 n=4+5) CacheHitProc/size=4096-4 61.6ns ± 1% 69.7ns ± 5% +13.29% (p=0.008 n=5+5) name old alloc/op new alloc/op delta NoopStorage-4 0.00B 0.00B ~ (all equal) CacheStartup-4 269B ± 0% 285B ± 0% +5.95% (p=0.008 n=5+5) CacheNoHit/size=0-4 225B ± 0% 153B ± 0% -32.12% (p=0.008 n=5+5) CacheNoHit/size=16-4 225B ± 0% 153B ± 0% -32.00% (p=0.029 n=4+4) CacheNoHit/size=128-4 225B ± 1% 153B ± 0% -31.76% (p=0.008 n=5+5) CacheNoHit/size=512-4 225B ± 1% 154B ± 0% -31.50% (p=0.008 n=5+5) CacheNoHit/size=4096-4 224B ± 0% 155B ± 0% -30.80% (p=0.008 n=5+5) CacheHit/size=0-4 0.00B 13.40B ±42% +Inf% (p=0.008 n=5+5) CacheHit/size=16-4 0.00B 0.00B ~ (all equal) CacheHit/size=128-4 0.00B 0.00B ~ (all equal) CacheHit/size=512-4 0.00B 0.00B ~ (all equal) CacheHit/size=4096-4 0.00B 0.00B ~ (all equal) NoopStoragePar-4 0.00B 0.00B ~ (all equal) CacheStartupPar-4 267B ± 0% 282B ± 1% +5.67% (p=0.016 n=4+5) CacheNoHitPar/size=0-4 232B ± 1% 162B ± 1% -30.11% (p=0.008 n=5+5) CacheNoHitPar/size=16-4 228B ± 1% 161B ± 0% -29.21% (p=0.008 n=5+5) CacheNoHitPar/size=128-4 229B ± 1% 162B ± 0% -29.43% (p=0.008 n=5+5) CacheNoHitPar/size=512-4 228B ± 1% 162B ± 1% -28.86% (p=0.008 n=5+5) CacheNoHitPar/size=4096-4 224B ± 0% 166B ± 0% -26.02% (p=0.000 n=5+4) CacheHitPar/size=0-4 1.00B ± 0% 13.60B ± 4% +1260.00% (p=0.008 n=5+5) CacheHitPar/size=16-4 0.00B 0.00B ~ (all equal) CacheHitPar/size=128-4 0.00B 0.00B ~ (all equal) CacheHitPar/size=512-4 0.00B 0.00B ~ (all equal) CacheHitPar/size=4096-4 0.00B 0.00B ~ (all equal) NoopStorageProc-4 0.00B 0.00B ~ (all equal) CacheStartupProc-4 269B ± 0% 285B ± 0% +5.95% (p=0.008 n=5+5) CacheNoHitProc/size=0-4 240B ± 0% 194B ± 0% -19.17% (p=0.000 n=5+4) CacheNoHitProc/size=16-4 240B ± 2% 194B ± 1% -19.38% (p=0.008 n=5+5) CacheNoHitProc/size=128-4 241B ± 0% 193B ± 1% -20.00% (p=0.016 n=4+5) CacheNoHitProc/size=512-4 241B ± 1% 188B ± 2% -22.06% (p=0.008 n=5+5) CacheNoHitProc/size=4096-4 240B ± 1% 179B ± 0% -25.52% (p=0.008 n=5+5) CacheHitProc/size=0-4 0.00B 3.60B ±17% +Inf% (p=0.008 n=5+5) CacheHitProc/size=16-4 0.00B 0.00B ~ (all equal) CacheHitProc/size=128-4 0.00B 0.00B ~ (all equal) CacheHitProc/size=512-4 0.00B 0.00B ~ (all equal) CacheHitProc/size=4096-4 0.00B 0.00B ~ (all equal) name old allocs/op new allocs/op delta NoopStorage-4 0.00 0.00 ~ (all equal) CacheStartup-4 5.00 ± 0% 5.00 ± 0% ~ (all equal) CacheNoHit/size=0-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHit/size=16-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHit/size=128-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHit/size=512-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHit/size=4096-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheHit/size=0-4 0.00 0.00 ~ (all equal) CacheHit/size=16-4 0.00 0.00 ~ (all equal) CacheHit/size=128-4 0.00 0.00 ~ (all equal) CacheHit/size=512-4 0.00 0.00 ~ (all equal) CacheHit/size=4096-4 0.00 0.00 ~ (all equal) NoopStoragePar-4 0.00 0.00 ~ (all equal) CacheStartupPar-4 4.00 ± 0% 4.00 ± 0% ~ (all equal) CacheNoHitPar/size=0-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitPar/size=16-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitPar/size=128-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitPar/size=512-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitPar/size=4096-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheHitPar/size=0-4 0.00 0.00 ~ (all equal) CacheHitPar/size=16-4 0.00 0.00 ~ (all equal) CacheHitPar/size=128-4 0.00 0.00 ~ (all equal) CacheHitPar/size=512-4 0.00 0.00 ~ (all equal) CacheHitPar/size=4096-4 0.00 0.00 ~ (all equal) NoopStorageProc-4 0.00 0.00 ~ (all equal) CacheStartupProc-4 5.00 ± 0% 5.00 ± 0% ~ (all equal) CacheNoHitProc/size=0-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitProc/size=16-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitProc/size=128-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitProc/size=512-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitProc/size=4096-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheHitProc/size=0-4 0.00 0.00 ~ (all equal) CacheHitProc/size=16-4 0.00 0.00 ~ (all equal) CacheHitProc/size=128-4 0.00 0.00 ~ (all equal) CacheHitProc/size=512-4 0.00 0.00 ~ (all equal) CacheHitProc/size=4096-4 0.00 0.00 ~ (all equal)

X zodb/cache: Also free OCE entries on GC
Until now we were only GC'ing RCE entries, but OCE entries (top-level cache entry for an OID) were never GC'ed thus allowing cache size to grow infinitely. Even though the average N(allocations) stays the same this change lowers pressure on amount of memory allocated and thus helping avoid some GC speeds things up (see attachment). Notes: 1. Hit*/size=0 increase is probably due to Cache.entryMap going to empty/1-element back and forth constantly. size/0 is not going to happen in practice and so this wat we can accept regression here. ( still NoHit*/size=0 works faster ). 2. HitProc/size=* "regression should be within noise" name old time/op new time/op delta NoopStorage-4 56.6ns ± 1% 56.2ns ± 0% ~ (p=0.079 n=5+5) CacheStartup-4 1.24µs ± 6% 1.28µs ± 9% ~ (p=0.595 n=5+5) CacheNoHit/size=0-4 1.44µs ± 4% 0.93µs ± 2% -35.26% (p=0.008 n=5+5) CacheNoHit/size=16-4 1.43µs ± 5% 0.92µs ± 1% -35.19% (p=0.008 n=5+5) CacheNoHit/size=128-4 1.45µs ± 3% 0.94µs ± 1% -35.03% (p=0.008 n=5+5) CacheNoHit/size=512-4 1.44µs ± 4% 0.97µs ± 2% -32.64% (p=0.008 n=5+5) CacheNoHit/size=4096-4 1.45µs ± 2% 1.07µs ± 0% -25.85% (p=0.008 n=5+5) CacheHit/size=0-4 131ns ± 2% 276ns ±22% +110.99% (p=0.008 n=5+5) CacheHit/size=16-4 122ns ± 1% 121ns ± 1% ~ (p=0.079 n=5+5) CacheHit/size=128-4 126ns ± 2% 125ns ± 1% ~ (p=0.563 n=5+5) CacheHit/size=512-4 127ns ± 1% 126ns ± 0% ~ (p=0.095 n=5+4) CacheHit/size=4096-4 128ns ± 0% 128ns ± 0% ~ (p=0.556 n=5+4) NoopStoragePar-4 30.6ns ± 4% 31.2ns ±10% ~ (p=0.690 n=5+5) CacheStartupPar-4 1.44µs ± 5% 1.43µs ± 3% ~ (p=0.690 n=5+5) CacheNoHitPar/size=0-4 1.62µs ± 4% 1.04µs ± 1% -35.76% (p=0.008 n=5+5) CacheNoHitPar/size=16-4 1.65µs ± 4% 1.05µs ± 1% -36.39% (p=0.008 n=5+5) CacheNoHitPar/size=128-4 1.64µs ± 5% 1.05µs ± 1% -35.84% (p=0.008 n=5+5) CacheNoHitPar/size=512-4 1.62µs ± 3% 1.08µs ± 1% -33.10% (p=0.008 n=5+5) CacheNoHitPar/size=4096-4 1.68µs ± 1% 1.18µs ± 0% -29.44% (p=0.008 n=5+5) CacheHitPar/size=0-4 215ns ± 0% 383ns ± 2% +78.23% (p=0.008 n=5+5) CacheHitPar/size=16-4 214ns ± 2% 214ns ± 0% ~ (p=0.786 n=5+5) CacheHitPar/size=128-4 210ns ± 0% 209ns ± 0% ~ (p=0.079 n=5+5) CacheHitPar/size=512-4 207ns ± 0% 206ns ± 0% -0.48% (p=0.008 n=5+5) CacheHitPar/size=4096-4 204ns ± 0% 202ns ± 0% -0.98% (p=0.000 n=5+4) NoopStorageProc-4 31.4ns ± 7% 33.7ns ± 5% ~ (p=0.151 n=5+5) CacheStartupProc-4 1.13µs ± 5% 1.12µs ± 3% ~ (p=0.690 n=5+5) CacheNoHitProc/size=0-4 1.12µs ± 5% 0.62µs ± 1% -44.52% (p=0.008 n=5+5) CacheNoHitProc/size=16-4 1.14µs ± 6% 0.63µs ± 1% -45.14% (p=0.008 n=5+5) CacheNoHitProc/size=128-4 1.06µs ± 5% 0.64µs ± 2% -40.12% (p=0.008 n=5+5) CacheNoHitProc/size=512-4 1.14µs ±11% 0.69µs ± 4% -39.87% (p=0.008 n=5+5) CacheNoHitProc/size=4096-4 1.14µs ± 9% 0.68µs ± 2% -40.21% (p=0.008 n=5+5) CacheHitProc/size=0-4 56.5ns ± 7% 84.6ns ±14% +49.66% (p=0.008 n=5+5) CacheHitProc/size=16-4 55.8ns ± 0% 62.0ns ± 6% +11.03% (p=0.008 n=5+5) CacheHitProc/size=128-4 56.6ns ± 0% 60.9ns ± 4% +7.63% (p=0.008 n=5+5) CacheHitProc/size=512-4 57.3ns ± 0% 64.1ns ± 7% +11.83% (p=0.016 n=4+5) CacheHitProc/size=4096-4 61.6ns ± 1% 69.7ns ± 5% +13.29% (p=0.008 n=5+5) name old alloc/op new alloc/op delta NoopStorage-4 0.00B 0.00B ~ (all equal) CacheStartup-4 269B ± 0% 285B ± 0% +5.95% (p=0.008 n=5+5) CacheNoHit/size=0-4 225B ± 0% 153B ± 0% -32.12% (p=0.008 n=5+5) CacheNoHit/size=16-4 225B ± 0% 153B ± 0% -32.00% (p=0.029 n=4+4) CacheNoHit/size=128-4 225B ± 1% 153B ± 0% -31.76% (p=0.008 n=5+5) CacheNoHit/size=512-4 225B ± 1% 154B ± 0% -31.50% (p=0.008 n=5+5) CacheNoHit/size=4096-4 224B ± 0% 155B ± 0% -30.80% (p=0.008 n=5+5) CacheHit/size=0-4 0.00B 13.40B ±42% +Inf% (p=0.008 n=5+5) CacheHit/size=16-4 0.00B 0.00B ~ (all equal) CacheHit/size=128-4 0.00B 0.00B ~ (all equal) CacheHit/size=512-4 0.00B 0.00B ~ (all equal) CacheHit/size=4096-4 0.00B 0.00B ~ (all equal) NoopStoragePar-4 0.00B 0.00B ~ (all equal) CacheStartupPar-4 267B ± 0% 282B ± 1% +5.67% (p=0.016 n=4+5) CacheNoHitPar/size=0-4 232B ± 1% 162B ± 1% -30.11% (p=0.008 n=5+5) CacheNoHitPar/size=16-4 228B ± 1% 161B ± 0% -29.21% (p=0.008 n=5+5) CacheNoHitPar/size=128-4 229B ± 1% 162B ± 0% -29.43% (p=0.008 n=5+5) CacheNoHitPar/size=512-4 228B ± 1% 162B ± 1% -28.86% (p=0.008 n=5+5) CacheNoHitPar/size=4096-4 224B ± 0% 166B ± 0% -26.02% (p=0.000 n=5+4) CacheHitPar/size=0-4 1.00B ± 0% 13.60B ± 4% +1260.00% (p=0.008 n=5+5) CacheHitPar/size=16-4 0.00B 0.00B ~ (all equal) CacheHitPar/size=128-4 0.00B 0.00B ~ (all equal) CacheHitPar/size=512-4 0.00B 0.00B ~ (all equal) CacheHitPar/size=4096-4 0.00B 0.00B ~ (all equal) NoopStorageProc-4 0.00B 0.00B ~ (all equal) CacheStartupProc-4 269B ± 0% 285B ± 0% +5.95% (p=0.008 n=5+5) CacheNoHitProc/size=0-4 240B ± 0% 194B ± 0% -19.17% (p=0.000 n=5+4) CacheNoHitProc/size=16-4 240B ± 2% 194B ± 1% -19.38% (p=0.008 n=5+5) CacheNoHitProc/size=128-4 241B ± 0% 193B ± 1% -20.00% (p=0.016 n=4+5) CacheNoHitProc/size=512-4 241B ± 1% 188B ± 2% -22.06% (p=0.008 n=5+5) CacheNoHitProc/size=4096-4 240B ± 1% 179B ± 0% -25.52% (p=0.008 n=5+5) CacheHitProc/size=0-4 0.00B 3.60B ±17% +Inf% (p=0.008 n=5+5) CacheHitProc/size=16-4 0.00B 0.00B ~ (all equal) CacheHitProc/size=128-4 0.00B 0.00B ~ (all equal) CacheHitProc/size=512-4 0.00B 0.00B ~ (all equal) CacheHitProc/size=4096-4 0.00B 0.00B ~ (all equal) name old allocs/op new allocs/op delta NoopStorage-4 0.00 0.00 ~ (all equal) CacheStartup-4 5.00 ± 0% 5.00 ± 0% ~ (all equal) CacheNoHit/size=0-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHit/size=16-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHit/size=128-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHit/size=512-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHit/size=4096-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheHit/size=0-4 0.00 0.00 ~ (all equal) CacheHit/size=16-4 0.00 0.00 ~ (all equal) CacheHit/size=128-4 0.00 0.00 ~ (all equal) CacheHit/size=512-4 0.00 0.00 ~ (all equal) CacheHit/size=4096-4 0.00 0.00 ~ (all equal) NoopStoragePar-4 0.00 0.00 ~ (all equal) CacheStartupPar-4 4.00 ± 0% 4.00 ± 0% ~ (all equal) CacheNoHitPar/size=0-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitPar/size=16-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitPar/size=128-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitPar/size=512-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitPar/size=4096-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheHitPar/size=0-4 0.00 0.00 ~ (all equal) CacheHitPar/size=16-4 0.00 0.00 ~ (all equal) CacheHitPar/size=128-4 0.00 0.00 ~ (all equal) CacheHitPar/size=512-4 0.00 0.00 ~ (all equal) CacheHitPar/size=4096-4 0.00 0.00 ~ (all equal) NoopStorageProc-4 0.00 0.00 ~ (all equal) CacheStartupProc-4 5.00 ± 0% 5.00 ± 0% ~ (all equal) CacheNoHitProc/size=0-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitProc/size=16-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitProc/size=128-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitProc/size=512-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheNoHitProc/size=4096-4 3.00 ± 0% 3.00 ± 0% ~ (all equal) CacheHitProc/size=0-4 0.00 0.00 ~ (all equal) CacheHitProc/size=16-4 0.00 0.00 ~ (all equal) CacheHitProc/size=128-4 0.00 0.00 ~ (all equal) CacheHitProc/size=512-4 0.00 0.00 ~ (all equal) CacheHitProc/size=4096-4 0.00 0.00 ~ (all equal)
521d3ae5 · Kirill Smelkov · 1f07e51a · 521d3ae5 · 521d3ae5
Commit 521d3ae5 authored Jan 03, 2018 by Kirill Smelkov
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Inline Side-by-side

Showing with 87 additions and 24 deletions

go/zodb/cache.go go/zodb/cache.go +34 -17

go/zodb/cache_test.go go/zodb/cache_test.go +53 -7

No files found.
--- a/go/zodb/cache.go
+++ b/go/zodb/cache.go
@@ -59,6 +59,8 @@ type Cache struct {

 // oidCacheEntry maintains cached revisions for 1 oid
 type oidCacheEntry struct {
+	oid Oid
+
 	sync.Mutex

 	// cached revisions in ascending order
@@ -174,7 +176,7 @@ func (c *Cache) Load(ctx context.Context, xid Xid) (buf *Buf, serial Tid, err er
 	} else {
 		// XXX use connection poll
 		// XXX or it should be cared by loader?
-		c.loadRCE(ctx, rce, xid.Oid)
+		c.loadRCE(ctx, rce)
 	}

 	if rce.err != nil {
@@ -200,7 +202,7 @@ func (c *Cache) Prefetch(ctx context.Context, xid Xid) {
 	// spawn loading in the background if rce was not yet loaded
 	if rceNew {
 		// XXX use connection poll
-		go c.loadRCE(ctx, rce, xid.Oid)
+		go c.loadRCE(ctx, rce)
 	}

 }
@@ -237,7 +239,7 @@ func (c *Cache) lookupRCE(xid Xid, wantBufRef int) (rce *revCacheEntry, rceNew b
 		c.mu.Lock()
 		oce = c.entryMap[xid.Oid]
 		if oce == nil {
-			oce = &oidCacheEntry{}
+			oce = &oidCacheEntry{oid: xid.Oid}
 			c.entryMap[xid.Oid] = oce
 		}
 		cacheHead = c.head // reload c.head because we relocked the cache
@@ -306,9 +308,9 @@ func (c *Cache) lookupRCE(xid Xid, wantBufRef int) (rce *revCacheEntry, rceNew b
 //
 // rce must be new just created by lookupRCE() with returned rceNew=true.
 // loading completion is signalled by marking rce.ready done.
-func (c *Cache) loadRCE(ctx context.Context, rce *revCacheEntry, oid Oid) {
+func (c *Cache) loadRCE(ctx context.Context, rce *revCacheEntry) {
 	oce := rce.parent
-	buf, serial, err := c.loader.Load(ctx, Xid{At: rce.head, Oid: oid})
+	buf, serial, err := c.loader.Load(ctx, Xid{At: rce.head, Oid: oce.oid})

 	// normalize buf/serial if it was error
 	if err != nil {
@@ -322,7 +324,7 @@ func (c *Cache) loadRCE(ctx context.Context, rce *revCacheEntry, oid Oid) {
 	rce.err = err
 	// verify db gives serial <= head
 	if rce.serial > rce.head {
-		rce.markAsDBError(oid, "load(@%v) -> %v", rce.head, serial)
+		rce.markAsDBError("load(@%v) -> %v", rce.head, serial)
 	}

 	δsize := rce.buf.Len()
@@ -365,7 +367,7 @@ func (c *Cache) loadRCE(ctx context.Context, rce *revCacheEntry, oid Oid) {
 	rceDropped := false
 	if i + 1 < len(oce.rcev) {
 		rceNext := oce.rcev[i+1]
-		if rceNext.loaded() && tryMerge(rce, rceNext, rce, oid) {
+		if rceNext.loaded() && tryMerge(rce, rceNext, rce) {
 			// not δsize -= len(rce.buf.Data)
 			// tryMerge can change rce.buf if consistency is broken
 			δsize = 0
@@ -380,7 +382,7 @@ func (c *Cache) loadRCE(ctx context.Context, rce *revCacheEntry, oid Oid) {
 	var rcePrevDropped *revCacheEntry
 	if i > 0 {
 		rcePrev := oce.rcev[i-1]
-		if rcePrev.loaded() && tryMerge(rcePrev, rce, rce, oid) {
+		if rcePrev.loaded() && tryMerge(rcePrev, rce, rce) {
 			rcePrevDropped = rcePrev
 			if rcePrev.accounted {
 				δsize -= rcePrev.buf.Len()
@@ -433,9 +435,7 @@ func (c *Cache) loadRCE(ctx context.Context, rce *revCacheEntry, oid Oid) {
 // return: true if merging done and thus prev was dropped from parent
 //
 // must be called with .parent locked
-//
-// XXX move oid from args to revCacheEntry?
-func tryMerge(prev, next, cur *revCacheEntry, oid Oid) bool {
+func tryMerge(prev, next, cur *revCacheEntry) bool {

 	//		  can merge if    consistent if
 	//	                          (if merging)
@@ -458,12 +458,12 @@ func tryMerge(prev, next, cur *revCacheEntry, oid Oid) bool {
 		// check consistency
 		switch {
 		case prev.err == nil && prev.serial != next.serial:
-			cur.markAsDBError(oid, "load(@%v) -> %v; load(@%v) -> %v",
+			cur.markAsDBError("load(@%v) -> %v; load(@%v) -> %v",
 				prev.head, prev.serial, next.head, next.serial)

 		case prev.err != nil && !isErrNoData(prev.err):
 			if cur.err == nil {
-				cur.markAsDBError(oid, "load(@%v) -> %v; load(@%v) -> %v",
+				cur.markAsDBError("load(@%v) -> %v; load(@%v) -> %v",
 					prev.head, prev.err, next.head, next.serial)
 			}
 		}
@@ -539,11 +539,16 @@ func (c *Cache) gc() {

 		rce := h.rceFromInLRU()
 		oce := rce.parent
+		oceFree := false // whether to GC whole rce.parent OCE cache entry

 		oce.Lock()
 		i := oce.find(rce)
 		if i != -1 {	// rce could be already deleted by e.g. merge
 			oce.deli(i)
+			if len(oce.rcev) == 0 {
+				oceFree = true
+			}
+
 			c.size -= rce.buf.Len()
 			//fmt.Printf("gc: free %d bytes\n", rce.buf.Len()))

@@ -558,6 +563,18 @@ func (c *Cache) gc() {

 		h.Delete()
 		c.gcMu.Unlock()
+
+		if oceFree {
+			c.mu.Lock()
+			oce.Lock()
+			// recheck once again oce is still not used
+			// (it could be looked up again in the meantime we were not holding its lock)
+			if len(oce.rcev) == 0 {
+				delete(c.entryMap, oce.oid)
+			}
+			oce.Unlock()
+			c.mu.Unlock()
+		}
 	}
 }

@@ -620,7 +637,7 @@ func (oce *oidCacheEntry) deli(i int) {
 	oce.rcev = oce.rcev[:n]
 }

-// del delets rce from .rcev.
+// del deletes rce from .rcev.
 // it panics if rce is not there.
 //
 // oce must be locked.
@@ -684,9 +701,9 @@ func errDB(oid Oid, format string, argv ...interface{}) error {
 //
 // Caller must be the only one to access rce.
 // In practice this means rce was just loaded but neither yet signalled to be
-// ready to waiter, nor yet made visible to GC (via adding to Cacle.lru list).
-func (rce *revCacheEntry) markAsDBError(oid Oid, format string, argv ...interface{}) {
-	rce.err = errDB(oid, format, argv...)
+// ready to waiter, nor yet made visible to GC (via adding to Cache.lru list).
+func (rce *revCacheEntry) markAsDBError(format string, argv ...interface{}) {
+	rce.err = errDB(rce.parent.oid, format, argv...)
 	rce.buf.XRelease()
 	rce.buf = nil
 	rce.serial = 0

--- a/go/zodb/cache_test.go
+++ b/go/zodb/cache_test.go
@@ -170,6 +170,23 @@ func TestCache(t *testing.T) {
 		}
 	}

+	checkOIDV := func(oidvOk ...Oid) {
+		t.Helper()
+
+		var oidv []Oid
+		for oid := range c.entryMap {
+			oidv = append(oidv, oid)
+		}
+
+		sort.Slice(oidv, func(i, j int) bool {
+			return oidv[i] < oidv[j]
+		})
+
+		if !reflect.DeepEqual(oidv, oidvOk) {
+			t.Fatalf("oidv: %s", pretty.Compare(oidvOk, oidv))
+		}
+	}
+
 	checkRCE := func(rce *revCacheEntry, head, serial Tid, buf *Buf, err error) {
 		t.Helper()
 		bad := &bytes.Buffer{}
@@ -193,7 +210,10 @@ func TestCache(t *testing.T) {

 	checkOCE := func(oid Oid, rcev ...*revCacheEntry) {
 		t.Helper()
-		oce := c.entryMap[oid]
+		oce, ok := c.entryMap[oid]
+		if !ok {
+			t.Fatalf("oce(%v): not present in cache", oid)
+		}
 		oceRcev := oce.rcev
 		if len(oceRcev) == 0 {
 			oceRcev = nil // nil != []{}
@@ -233,10 +253,12 @@ func TestCache(t *testing.T) {
 	// ---- verify cache behaviour for must be loaded/merged entries ----
 	// (this exercises mostly loadRCE/tryMerge)

+	checkOIDV()
 	checkMRU(0)

 	// load @2 -> new rce entry
 	checkLoad(xidat(1,2), nil, 0, &ErrXidMissing{xidat(1,2)})
+	checkOIDV(1)
 	oce1 := c.entryMap[1]
 	ok1(len(oce1.rcev) == 1)
 	rce1_h2 := oce1.rcev[0]
@@ -245,6 +267,7 @@ func TestCache(t *testing.T) {

 	// load @3 -> 2] merged with 3]
 	checkLoad(xidat(1,3), nil, 0, &ErrXidMissing{xidat(1,3)})
+	checkOIDV(1)
 	ok1(len(oce1.rcev) == 1)
 	rce1_h3 := oce1.rcev[0]
 	ok1(rce1_h3 != rce1_h2) // rce1_h2 was merged into rce1_h3
@@ -253,6 +276,7 @@ func TestCache(t *testing.T) {

 	// load @1 -> 1] merged with 3]
 	checkLoad(xidat(1,1), nil, 0, &ErrXidMissing{xidat(1,1)})
+	checkOIDV(1)
 	ok1(len(oce1.rcev) == 1)
 	ok1(oce1.rcev[0] == rce1_h3)
 	checkRCE(rce1_h3, 3, 0, nil, &ErrXidMissing{xidat(1,3)})
@@ -260,6 +284,7 @@ func TestCache(t *testing.T) {

 	// load @5 -> new rce entry with data
 	checkLoad(xidat(1,5), b(hello), 4, nil)
+	checkOIDV(1)
 	ok1(len(oce1.rcev) == 2)
 	rce1_h5 := oce1.rcev[1]
 	checkRCE(rce1_h5, 5, 4, b(hello), nil)
@@ -268,11 +293,13 @@ func TestCache(t *testing.T) {

 	// load @4 -> 4] merged with 5]
 	checkLoad(xidat(1,4), b(hello), 4, nil)
+	checkOIDV(1)
 	checkOCE(1, rce1_h3, rce1_h5)
 	checkMRU(5, rce1_h5, rce1_h3)

 	// load @6 -> 5] merged with 6]
 	checkLoad(xidat(1,6), b(hello), 4, nil)
+	checkOIDV(1)
 	ok1(len(oce1.rcev) == 2)
 	rce1_h6 := oce1.rcev[1]
 	ok1(rce1_h6 != rce1_h5)
@@ -282,6 +309,7 @@ func TestCache(t *testing.T) {

 	// load @7 -> ioerr + new rce
 	checkLoad(xidat(1,7), nil, 0, ioerr)
+	checkOIDV(1)
 	ok1(len(oce1.rcev) == 3)
 	rce1_h7 := oce1.rcev[2]
 	checkRCE(rce1_h7, 7, 0, nil, ioerr)
@@ -290,6 +318,7 @@ func TestCache(t *testing.T) {

 	// load @9 -> ioerr + new rce (IO errors are not merged)
 	checkLoad(xidat(1,9), nil, 0, ioerr)
+	checkOIDV(1)
 	ok1(len(oce1.rcev) == 4)
 	rce1_h9 := oce1.rcev[3]
 	checkRCE(rce1_h9, 9, 0, nil, ioerr)
@@ -298,6 +327,7 @@ func TestCache(t *testing.T) {

 	// load @10 -> new data rce, not merged with ioerr at 9]
 	checkLoad(xidat(1,10), b(world), 10, nil)
+	checkOIDV(1)
 	ok1(len(oce1.rcev) == 5)
 	rce1_h10 := oce1.rcev[4]
 	checkRCE(rce1_h10, 10, 10, b(world), nil)
@@ -306,6 +336,7 @@ func TestCache(t *testing.T) {

 	// load @11 -> 10] merged with 11]
 	checkLoad(xidat(1,11), b(world), 10, nil)
+	checkOIDV(1)
 	ok1(len(oce1.rcev) == 5)
 	rce1_h11 := oce1.rcev[4]
 	ok1(rce1_h11 != rce1_h10)
@@ -323,6 +354,7 @@ func TestCache(t *testing.T) {
 	rce1_h15.serial = 10
 	rce1_h15.buf = mkbuf(world)
 	// here: first half of loadRCE(15]) before close(15].ready)
+	checkOIDV(1)
 	checkOCE(1, rce1_h3, rce1_h6, rce1_h7, rce1_h9, rce1_h11, rce1_h15)
 	ok1(!rce1_h15.loaded())
 	checkMRU(12, rce1_h11, rce1_h9, rce1_h7, rce1_h6, rce1_h3) // no 15] yet
@@ -331,6 +363,7 @@ func TestCache(t *testing.T) {
 	//  automatically at lookup phase)
 	rce1_h13, new13 := c.lookupRCE(xidat(1,13), +0)
 	ok1(new13)
+	checkOIDV(1)
 	checkOCE(1, rce1_h3, rce1_h6, rce1_h7, rce1_h9, rce1_h11, rce1_h13, rce1_h15)
 	checkMRU(12, rce1_h11, rce1_h9, rce1_h7, rce1_h6, rce1_h3) // no <14 and <16 yet

@@ -338,13 +371,15 @@ func TestCache(t *testing.T) {
 	rce1_h15.waitBufRef = -1
 	rce1_h15.ready.Done()
 	ok1(rce1_h15.loaded())
+	checkOIDV(1)
 	checkOCE(1, rce1_h3, rce1_h6, rce1_h7, rce1_h9, rce1_h11, rce1_h13, rce1_h15)
 	checkMRU(12, rce1_h11, rce1_h9, rce1_h7, rce1_h6, rce1_h3) // no 13] and 15] yet

 	// (13] also becomes ready and takes oce lock first, merging 11] and 13] into 15].
 	//  15] did not yet took oce lock so c.size is temporarily reduced and
 	//  15] is not yet on LRU list)
-	c.loadRCE(ctx, rce1_h13, 1)
+	c.loadRCE(ctx, rce1_h13)
+	checkOIDV(1)
 	checkRCE(rce1_h13, 13, 10, b(world), nil)
 	checkRCE(rce1_h15, 15, 10, b(world), nil)
 	checkRCE(rce1_h11, 11, 10, b(world), nil)
@@ -353,7 +388,8 @@ func TestCache(t *testing.T) {

 	// (15] takes oce lock and updates c.size and LRU list)
 	rce1_h15.ready.Add(1) // so loadRCE could run
-	c.loadRCE(ctx, rce1_h15, 1)
+	c.loadRCE(ctx, rce1_h15)
+	checkOIDV(1)
 	checkOCE(1, rce1_h3, rce1_h6, rce1_h7, rce1_h9, rce1_h15)
 	checkMRU(12, rce1_h15, rce1_h9, rce1_h7, rce1_h6, rce1_h3)

@@ -370,11 +406,13 @@ func TestCache(t *testing.T) {
 	rce1_h16.waitBufRef = -1
 	rce1_h16.ready.Done()
 	ok1(rce1_h16.loaded())
+	checkOIDV(1)
 	checkOCE(1, rce1_h3, rce1_h6, rce1_h7, rce1_h9, rce1_h15, rce1_h16, rce1_h17)
 	checkMRU(12, rce1_h15, rce1_h9, rce1_h7, rce1_h6, rce1_h3) // no 16] and 17] yet

 	// (17] loads and takes oce lock first - merge 16] with 17])
-	c.loadRCE(ctx, rce1_h17, 1)
+	c.loadRCE(ctx, rce1_h17)
+	checkOIDV(1)
 	checkRCE(rce1_h17, 17, 16, b(zz), nil)
 	checkRCE(rce1_h16, 16, 16, b(zz), nil)
 	checkOCE(1, rce1_h3, rce1_h6, rce1_h7, rce1_h9, rce1_h15, rce1_h17)
@@ -385,6 +423,7 @@ func TestCache(t *testing.T) {
 	ok1(len(oce1.rcev) == 6)
 	rce1_h19 := oce1.rcev[5]
 	ok1(rce1_h19 != rce1_h17)
+	checkOIDV(1)
 	checkRCE(rce1_h19, 19, 16, b(zz), nil)
 	checkOCE(1,  rce1_h3, rce1_h6, rce1_h7, rce1_h9, rce1_h15, rce1_h19)
 	checkMRU(14, rce1_h19, rce1_h15, rce1_h9, rce1_h7, rce1_h6, rce1_h3)
@@ -393,6 +432,7 @@ func TestCache(t *testing.T) {
 	checkLoad(xidat(1,20), b(www), 20, nil)
 	ok1(len(oce1.rcev) == 7)
 	rce1_h20 := oce1.rcev[6]
+	checkOIDV(1)
 	checkRCE(rce1_h20, 20, 20, b(www), nil)
 	checkOCE(1,  rce1_h3, rce1_h6, rce1_h7, rce1_h9, rce1_h15, rce1_h19, rce1_h20)
 	checkMRU(17, rce1_h20, rce1_h19, rce1_h15, rce1_h9, rce1_h7, rce1_h6, rce1_h3)
@@ -402,6 +442,7 @@ func TestCache(t *testing.T) {
 	ok1(len(oce1.rcev) == 7)
 	rce1_h21 := oce1.rcev[6]
 	ok1(rce1_h21 != rce1_h20)
+	checkOIDV(1)
 	checkRCE(rce1_h21, 21, 20, b(www), nil)
 	checkOCE(1,  rce1_h3, rce1_h6, rce1_h7, rce1_h9, rce1_h15, rce1_h19, rce1_h21)
 	checkMRU(17, rce1_h21, rce1_h19, rce1_h15, rce1_h9, rce1_h7, rce1_h6, rce1_h3)
@@ -439,7 +480,8 @@ func TestCache(t *testing.T) {
 	// 8] must be separate from 7] and 9] because it is IO error there
 	rce1_h8, new8 := c.lookupRCE(xidat(1,8), +0)
 	ok1(new8)
-	c.loadRCE(ctx, rce1_h8, 1)
+	c.loadRCE(ctx, rce1_h8)
+	checkOIDV(1)
 	checkRCE(rce1_h8, 8, 0, nil, ioerr)
 	checkOCE(1, rce1_h3, rce1_h6, rce1_h7, rce1_h8, rce1_h9, rce1_h15, rce1_h19, rce1_h21)
 	checkMRU(17, rce1_h8, rce1_h21, rce1_h19, rce1_h15, rce1_h9, rce1_h7, rce1_h6, rce1_h3)
@@ -488,6 +530,7 @@ func TestCache(t *testing.T) {
 	gcstart  := &evCacheGCStart{c}
 	gcfinish := &evCacheGCFinish{c}

+	checkOIDV(1)
 	checkOCE(1, rce1_h3, rce1_h6, rce1_h7, rce1_h8, rce1_h9, rce1_h15, rce1_h19, rce1_h21)
 	checkMRU(17, rce1_h15, rce1_h6, rce1_h8, rce1_h21, rce1_h19, rce1_h9, rce1_h7, rce1_h3)

@@ -499,6 +542,7 @@ func TestCache(t *testing.T) {
 	// - 7]  (lru.2, ioerr, size=0)
 	// - 9]  (lru.3, ioerr, size=0)
 	// - 19] (lru.4, zz, size=2)
+	checkOIDV(1)
 	checkOCE(1,  rce1_h6, rce1_h8, rce1_h15, rce1_h21)
 	checkMRU(15, rce1_h15, rce1_h6, rce1_h8, rce1_h21)

@@ -511,6 +555,7 @@ func TestCache(t *testing.T) {
 	ok1(len(oce1.rcev) == 4)
 	rce1_h19_2 := oce1.rcev[3]
 	ok1(rce1_h19_2 != rce1_h19)
+	checkOIDV(1)
 	checkRCE(rce1_h19_2, 19, 16, b(zz), nil)
 	checkOCE(1,  rce1_h6, rce1_h8, rce1_h15, rce1_h19_2)
 	checkMRU(14, rce1_h19_2, rce1_h15, rce1_h6, rce1_h8)
@@ -525,6 +570,7 @@ func TestCache(t *testing.T) {
 	// - loaded  77] (big, size=10)
 	ok1(len(oce1.rcev) == 2)
 	rce1_h77 := oce1.rcev[1]
+	checkOIDV(1)
 	checkRCE(rce1_h77, 77, 77, b(big), nil)
 	checkOCE(1,  rce1_h19_2, rce1_h77)
 	checkMRU(12, rce1_h77, rce1_h19_2)
@@ -532,7 +578,7 @@ func TestCache(t *testing.T) {
 	// sizeMax=0 evicts everything from cache
 	go c.SetSizeMax(0)
 	tc.Expect(gcstart, gcfinish)
-	checkOCE(1)
+	checkOIDV()
 	checkMRU(0)

 	// and still loading works (because even if though rce's are evicted
@@ -540,7 +586,7 @@ func TestCache(t *testing.T) {

 	checkLoad(xidat(1,4), b(hello), 4, nil)
 	tc.Expect(gcstart, gcfinish)
-	checkOCE(1)
+	checkOIDV()
 	checkMRU(0)

 	// ---- Load vs concurrent GC ----