Model

Optimistic against pessimistic

If conflicts are rare, a lock manager is a cost paid on every operation to protect against something that almost never happens.

k \cdot c_{lock} (locking, every time) vs p_{conflict} \cdot work (optimistic, on conflict)

$k$: Items; items per transaction
$c_{lock}$: Lock cost; cost of one lock acquisition
$p_{conflict}$: Conflict probability; chance a transaction must redo its work

Trade-off: Coupling vs. coordination cost

How it behaves

One input on the horizontal axis, the others held fixed.

lockingoptimistic

Data table

Cost per transaction by conflict probability (k = 10, 150 ns per lock, 100 µs of work)
Conflict probability p	series	Cost per transaction
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.001	locking	1.5 µs
0.002	locking	1.5 µs
0.002	locking	1.5 µs
0.002	locking	1.5 µs
0.002	locking	1.5 µs
0.002	locking	1.5 µs
0.003	locking	1.5 µs
0.003	locking	1.5 µs
0.003	locking	1.5 µs
0.003	locking	1.5 µs
0.004	locking	1.5 µs
0.004	locking	1.5 µs
0.004	locking	1.5 µs
0.005	locking	1.5 µs
0.005	locking	1.5 µs
0.006	locking	1.5 µs
0.007	locking	1.5 µs
0.007	locking	1.5 µs
0.008	locking	1.5 µs
0.009	locking	1.5 µs
0.01	locking	1.5 µs
0.011	locking	1.5 µs
0.012	locking	1.5 µs
0.013	locking	1.5 µs
0.014	locking	1.5 µs
0.015	locking	1.5 µs
0.017	locking	1.5 µs
0.019	locking	1.5 µs
0.021	locking	1.5 µs
0.023	locking	1.5 µs
0.025	locking	1.5 µs
0.027	locking	1.5 µs
0.03	locking	1.5 µs
0.033	locking	1.5 µs
0.036	locking	1.5 µs
0.04	locking	1.5 µs
0.044	locking	1.5 µs
0.048	locking	1.5 µs
0.053	locking	1.5 µs
0.058	locking	1.5 µs
0.064	locking	1.5 µs
0.07	locking	1.5 µs
0.077	locking	1.5 µs
0.085	locking	1.5 µs
0.094	locking	1.5 µs
0.103	locking	1.5 µs
0.113	locking	1.5 µs
0.124	locking	1.5 µs
0.137	locking	1.5 µs
0.15	locking	1.5 µs
0.165	locking	1.5 µs
0.182	locking	1.5 µs
0.2	locking	1.5 µs
0	optimistic	0.01 µs
0	optimistic	0.011 µs
0	optimistic	0.012 µs
0	optimistic	0.013 µs
0	optimistic	0.015 µs
0	optimistic	0.016 µs
0	optimistic	0.018 µs
0	optimistic	0.019 µs
0	optimistic	0.021 µs
0	optimistic	0.024 µs
0	optimistic	0.026 µs
0	optimistic	0.028 µs
0	optimistic	0.031 µs
0	optimistic	0.034 µs
0	optimistic	0.038 µs
0	optimistic	0.042 µs
0	optimistic	0.046 µs
0.001	optimistic	0.05 µs
0.001	optimistic	0.055 µs
0.001	optimistic	0.061 µs
0.001	optimistic	0.067 µs
0.001	optimistic	0.074 µs
0.001	optimistic	0.081 µs
0.001	optimistic	0.089 µs
0.001	optimistic	0.098 µs
0.001	optimistic	0.108 µs
0.001	optimistic	0.118 µs
0.001	optimistic	0.13 µs
0.001	optimistic	0.143 µs
0.002	optimistic	0.157 µs
0.002	optimistic	0.173 µs
0.002	optimistic	0.19 µs
0.002	optimistic	0.209 µs
0.002	optimistic	0.23 µs
0.003	optimistic	0.253 µs
0.003	optimistic	0.278 µs
0.003	optimistic	0.306 µs
0.003	optimistic	0.336 µs
0.004	optimistic	0.37 µs
0.004	optimistic	0.407 µs
0.004	optimistic	0.447 µs
0.005	optimistic	0.492 µs
0.005	optimistic	0.541 µs
0.006	optimistic	0.595 µs
0.007	optimistic	0.654 µs
0.007	optimistic	0.719 µs
0.008	optimistic	0.791 µs
0.009	optimistic	0.87 µs
0.01	optimistic	0.956 µs
0.011	optimistic	1.05 µs
0.012	optimistic	1.16 µs
0.013	optimistic	1.27 µs
0.014	optimistic	1.4 µs
0.015	optimistic	1.54 µs
0.017	optimistic	1.69 µs
0.019	optimistic	1.86 µs
0.021	optimistic	2.05 µs
0.023	optimistic	2.25 µs
0.025	optimistic	2.47 µs
0.027	optimistic	2.72 µs
0.03	optimistic	2.99 µs
0.033	optimistic	3.29 µs
0.036	optimistic	3.62 µs
0.04	optimistic	3.98 µs
0.044	optimistic	4.37 µs
0.048	optimistic	4.81 µs
0.053	optimistic	5.29 µs
0.058	optimistic	5.82 µs
0.064	optimistic	6.4 µs
0.07	optimistic	7.03 µs
0.077	optimistic	7.73 µs
0.085	optimistic	8.5 µs
0.094	optimistic	9.35 µs
0.103	optimistic	10.3 µs
0.113	optimistic	11.3 µs
0.124	optimistic	12.4 µs
0.137	optimistic	13.7 µs
0.15	optimistic	15 µs
0.165	optimistic	16.5 µs
0.182	optimistic	18.2 µs
0.2	optimistic	20 µs

Worked examples, recomputed

Every number the mechanisms quote for this relationship is recomputed from it, so none of them disagrees with it.

Where it appears

Each mechanism states it in its own names. The general symbol each name stands for is shown with it.

Transactions & Concurrency Control
What happens when many users change the same data at the same time, and how do we keep the result correct?
$k \times c_{lock} vs p_{conflict} \times work$
Read it in place →