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perf: store/cachekv: avoid a map lookup if unnecessary, clear maps fast, avoid keys sort #10486
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odeke-em
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orijtech:store-cachekv-reduce-unnecessary-conversions-and-memory-pressure-in-Store.Write
Nov 8, 2021
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I think this is incorrect in the general case. I've had to actually undo this in the CacheKV store myself in the Osmosis branch in a couple places.
This preserves the old capacity of the map, which makes iterating over map entries / sorting / etc. take much longer. (hashmap iteration time is proportional to its capacity, not its number of entries)
So in the Osmosis case, or a chain upgrading case, if you have one very large blcok, then many short blocks, every short block will take similar store time for iteration as the long block does.
What we did in the Osmosis branch was only use a map clearing idiom if the map is sufficiently small, otherwise we just clear it.
(This made a difference between N^2 behavior and O(N) behavior in our codebase, due to iterator things. Potentially less crucial here, but I'd still caution against this as a premature optimization)
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According to https://go-review.googlesource.com/c/go/+/110055/, the struct reinitialized and memory is cleared. So the optimization is that we save on heap memory allocation. However, if one TX will consume looooot of memory then we may have a problem (eg: huge iterator).
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@odeke-em can you help answer ^^