presto-main/src/main/java/io/prestosql/spiller/SpillSpaceTracker.java/SpillSpaceTracker/free
class SpillSpaceTracker:    public synchronized void free(long bytes)
    {
        checkArgument(bytes >= 0, "bytes is negative");
        checkArgument(currentBytes - bytes >= 0, "tried to free more disk space than is reserved");
        currentBytes -= bytes;
    }