8000 avoid unnecessary memory copy by wzk784533 · Pull Request #819 · google/leveldb · GitHub
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avoid unnecessary memory copy #819

Merged
merged 1 commit into from
Jul 13, 2020
Merged

avoid unnecessary memory copy #819

merged 1 commit into from
Jul 13, 2020

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wzk784533
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@wzk784533 wzk784533 commented Jul 11, 2020 •
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During the compaction phase, in this function

Status DBImpl::WriteLevel0Table(MemTable* mem, VersionEdit* edit,
                                Version* base)

It will then call

BuildTable(const std::string& dbname, Env* env, const Options& options,
                  TableCache* table_cache, Iterator* iter, FileMetaData* meta)

In BuildTable function, the code fragment is this:

TableBuilder* builder = new TableBuilder(options, file);
    meta->smallest.DecodeFrom(iter->key());
    for (; iter->Valid(); iter->Next()) {
      Slice key = iter->key();
      meta->largest.DecodeFrom(key);
      builder->Add(key, iter->value());
    }

In the iterator loop, call meta->largest.DecodeFrom(key) every time is unnecessary, which will copy every key to meta's largest field. We just need to copy the last key after the iteration is done.

bool DecodeFrom(const Slice& s) {
    rep_.assign(s.data(), s.size());
    return !rep_.empty();
  }

DecodeFrom just copy the slice to rep_. When there are many keys in Skiplist, and each key is a very long string, the wasting of CPU cycles are considerable.

pwnall
pwnall approved these changes Jul 13, 2020
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Great find. Thank you!

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pwnall commented Jul 13, 2020

I'll get this PR through our internal repository. It will get merged in a few days. Thank you very much for your contribution!

@pwnall pwnall merged commit c46e79c into google:master Jul 13, 2020
@reznikovl reznikovl mentioned this pull request Jan 20, 2023
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