When running on a cloud VM (n1-highcpu-96) with several NVMe SSDs and
some non-SSDs for tablebases, I noticed that the average SSD request size was
more than 256 kB. This doesn't make a lot of sense for Syzygy tablebases,
which have a block size of 32 bytes and very low locality.
Seemingly, the tablebase access patterns during probing make the OS,
at least Linux, think that readahead is advantageous; normally, it
gives up doing readahead if there are too many misses, but it doesn't,
perhaps due to the fairly high overall hit rates. (It seems the kernel cannot
distinguish between reading a block that was paged in because the userspace
wanted it explicitly, and one that was read as part of readahead.)
Setting MADV_RANDOM effectively turns off readahead, which causes
the request size to drop to 4 kB. In the aforemented cloud VM test,
this roughly tripled the amount of I/O requests that were able to go
through, while reducing the total traffic from 2.8 GB/sec to 56 MB/sec
(moving the bottleneck to the non-SSDs; it seems the SSDs could have
sustained many more requests).
Closes https://github.com/official-stockfish/Stockfish/pull/1829
No functional change.
There is no need to make this as large as 65536 just for the sake of the
single 7-man tablebase that happens to have the key 0xf9247fff. Idea for the
fix by Ronald de Man, who suggested simply to allow more buckets past the end.
We also implement Robin Hood hashing for the hash table, which takes the worst
-case search for full 7-man tablebases down from 68 to 11 probes (Also takes
the average probe length from 2.06 to 2.05). For a table with 8K entries, the
corresponding numbers would be worst-case from 9 to 4, with average from 1.30
to 1.29.
https://github.com/official-stockfish/Stockfish/pull/1747
No functional change
