page-cluster controls the number of pages up to which consecutive pages are read in from swap in a single attempt. This is the swap counterpart to page cache readahead. The mentioned consecutivity is not in terms of virtual/physical addresses, but consecutive on swap space - that means they were swapped out together.
It is a logarithmic value - setting it to zero means “1 page”, setting it to 1 means “2 pages”, setting it to 2 means “4 pages”, etc. Zero disables swap readahead completely.
The default value is three (eight pages at a time). There may be some small benefits in tuning this to a different value if your workload is swap-intensive.
Lower values mean lower latencies for initial faults, but at the same time extra faults and I/O delays for following faults if they would have been part of that consecutive pages readahead would have brought in.source