Allow Pin/UnpinBuffer to operate in a lockfree manner

Enterprise / PostgreSQL - Andres Freund [anarazel.de] - 10 April 2016 22:12 UTC

Pinning/Unpinning a buffer is a very frequent operation; especially in read-mostly cache resident workloads. Benchmarking shows that in various scenarios the spinlock protecting a buffer header's state becomes a significant bottleneck. The problem can be reproduced with pgbench -S on larger machines, but can be considerably worse for queries which touch the same buffers over and over at a high frequency (e.g. nested loops over a small inner table).

To allow atomic operations to be used, cram BufferDesc's flags, usage_count, buf_hdr_lock, refcount into a single 32bit atomic variable; that allows to manipulate them together using 32bit compare-and-swap operations. This requires reducing MAX_BACKENDS to 2^18-1 (which could be lifted by using a 64bit field, but it's not a realistic configuration atm).

As not all operations can easily implemented in a lockfree manner, implement the previous buf_hdr_lock via a flag bit in the atomic
variable. That way we can continue to lock the header in places where it's needed, but can get away without acquiring it in the more frequent hot-paths. There's some additional operations which can be done without the lock, but aren't in this patch; but the most important places are covered.

As bufmgr.c now essentially re-implements spinlocks, abstract the delay logic from s_lock.c into something more generic. It now has already two users, and more are coming up; there's a follupw patch for lwlock.c at least.

This patch is based on a proof-of-concept written by me, which Alexander Korotkov made into a fully working patch; the committed version is again revised by me. Benchmarking and testing has, amongst others, been provided by Dilip Kumar, Alexander Korotkov, Robert Haas.

On a large x86 system improvements for readonly pgbench, with a high client count, of a factor of 8 have been observed.

Author: Alexander Korotkov and Andres Freund Discussion: 2400449.GjM57CE0Yg@dinodell

4835458 Allow Pin/UnpinBuffer to operate in a lockfree manner.
contrib/pg_buffercache/pg_buffercache_pages.c | 15 +-
src/backend/storage/buffer/buf_init.c | 7 +-
src/backend/storage/buffer/bufmgr.c | 508 ++++++++++++++++---------
src/backend/storage/buffer/freelist.c | 44 ++-
src/backend/storage/buffer/localbuf.c | 64 +++-
src/backend/storage/lmgr/s_lock.c | 206 +++++-----
src/include/postmaster/postmaster.h | 15 +-
src/include/storage/buf_internals.h | 101 +++--
src/include/storage/s_lock.h | 18 +
src/tools/pgindent/typedefs.list | 1 +
10 files changed, 622 insertions(+), 357 deletions(-)

Upstream: git.postgresql.org

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