Introducing a parallel unzipping algorithm for pre-fetched buffers. Since we already know what buffers are going to be read, we can decompress a few of them in advance in an additional thread and give the impression that the data decompression comes for free (we gain up to 30% in reading intensive jobs).

The size of this unzipping cache is 20% the size of the TTreeCache and can be modified with TTreeCache::SetUnzipBufferSize(Long64_t bufferSize). Theoretically, we only need one buffer in advance but in practice we might fall short if the unzipping cache is too small (synchronization costs).

This experimental feature is disabled by default, to activate it use the static function TTreeCache::SetParallelUnzip(TTreeCacheUnzip::EParUnzipMode option = TTreeCacheUnzip::kEnable). The possible values to pass are: TTreeCacheUnzip::kEnable to enable it,TTreeCacheUnzip::kDisable to disable it and TTreeCacheUnzip::kForce to force it. Since it will only work if you have more than one core, the TTreeCacheUnzip::kForce option is useful to run it even if you have only one cpu (to measure the overhead, for example).


git-svn-id: http://root.cern.ch/svn/root/trunk@23685 27541ba8-7e3a-0410-8455-c3a389f83636