[question] any suggestion about compressing serialized data? about roaringformatspec

[lemire]

Do you get good results with gzip?

I have no experience compressing roaring bitmaps with generic codecs... assuredly, the impact will be data specific...

Related:

Compressing JSON: gzip vs zstd
https://lemire.me/blog/2021/06/30/compressing-json-gzip-vs-zstd/

[toien]

thanks for fast reply!

i write a test (java/golang), populate roaringbitmap with random uint32 and serialize it, also using gzip compress it, but it tunrns out data almost not compressed.

// populate with random data
Random r = new Random();
RoaringBitmap rbm = new RoaringBitmap();

for (int i = 0; i < size; i++) {
  long rValue = r.nextLong() & 0xffffffffL;
  int casted = (int) rValue;
  rbm.add(casted);
}

// dump to disk
ByteBuffer buffer = ByteBuffer.allocate(rbm.serializedSizeInBytes());
rbm.serialize(buffer);

Path path = Paths.get(filepath);

Files.write(path, buffer.array());

// compress and dump
Path cpath = Paths.get(compressedFilepath);

Files.write(cpath, new byte[0], StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING);

try (GZIPOutputStream gos = new GZIPOutputStream(new FileOutputStream(cpath.toFile()))) {
  gos.write(buffer.array());
}

here is result:

> ls -alht
-rw-r--r--   1 worker  staff    19M Mar 21 15:59 random-1000w.bin.gz
-rw-r--r--   1 worker  staff    20M Mar 21 15:59 random-1000w.bin

i am trying zstd

[toien]

it seems that generic compress not fit for roaringbitmap

INFO: lz4 decompressed len: 20501162, compressed len:20574769
INFO: zstd decompressed len: 20501162, compressed len:20415315

[lemire]

Interestingly, it looks like lz4 makes things worse in your test!

[derlaft]

RoaringFormatSpec : specification of the compressed-bitmap Roaring formats

roaring bitmaps are already a type of compression. therefore the entropy of the serialized data should already be rather close to the maximum (you can make an entropy graph for example using binwalk) and compressing it once more won't yield a significant result