[PATCH] Reduce LWLockWaitListLock() cache-line contention with adaptive spin reads

[Min, Baohong]

Hi Hackers,

The attached patch reduces cache-line contention in LWLockWaitListLock()'s 
spin-wait loop on large x86_64 systems. It does not change the locking 
primitive; it only changes how often spinning waiters re-read the shared lock 
state adaptively according to the contention situation on the fly, plus a small 
tweak to the spin-delay floor.

Problem:
On large x86_64 machines (e.g.: Intel Xeon, 2 sockets), pgbench TPC-B 
throughput was limited at high core and high client counts. We observed 
excessive time spent in LWLockWaitListLock() under high concurrency due to 
contention on the lock semantic. Perf profiles
show this is due to excessive cache-line bouncing between different cores and 
sockets.  We believe the better solution for this is to reduce contention 
adaptively. According to this idea, we drafted a patch and validated on server 
CPU as below.

Approach:
  1. In LWLockWaitListLock(), re-read lock->state only once every
     'spins_per_lock_read' spins instead of on every spin. The interval
     is adaptive: it grows (up to 256) under heavy contention and
     shrinks back toward 1 otherwise.
  2. In s_lock.c, lower MIN_DELAY_USEC from 1000us to 100us, which
     shortens the initial spin-delay back-off.


Benchmarks (pgbench, TPC-B-like, prepared protocol)
Intel Xeon, 60 cores, SMT-on, 2 sockets (240 vCPUs):

Clients        Baseline    Optimized      Speedup
8                  27878         28246               1.01
16                42851         44072               1.03
32                53893         54312               1.01
64                68807         72838               1.06
120              66825         74037               1.11
240              58585         75577               1.29
500              49208         74868               1.52
800              46982         74330               1.58
1000            44945         71436               1.59

Summary:
  - Low client counts (<= core count): no measurable impact. Throughput is
    unchanged within noise (1.01x-1.06x at 8-64 clients), so lightly loaded
    systems are unaffected.
  - High client counts (>= 120 clients): significant improvement. The patch
    delivers 1.11x-1.59x at 120-1000 clients.

Thanks,
Baohong Min
Intel DCG Software

Authors:  Min, Baohong <baohong....@intel.com>
                 Jin, Jun < jun.i....@intel.com>
                 Kim, Andrew <andrew....@intel.com>

LWLockWaitListLock-adaptive-spin-reads-v1.patch
Description: LWLockWaitListLock-adaptive-spin-reads-v1.patch