Re: Restructured Shared Buffer Hash Table

On Tue, Jul 7, 2026 at 8:14 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:

> > bufmgr.c also changed slightly to prevent a race condition.
>
> Hmm, we're now holding the buffer header lock much longer than before,
> in InvalidateBuffer(). It's a spinlock, it really should not be held for
> more than a few instructions. BufTableDelete() is very fast in the new

implementation, but still.

As Andres later clarified the problem is not the duration of the lock, is
the complexity of the safety risk of what we do under it.

Could we perhaps do some of
> BufTableDelete()'s work ahead of time, before we acquire the buffer
> header lock?

This is a better approach in my opinion, we can do the buffer search before
acquiring the spin-lock and just break the chain link under the spin-lock.
It is easier to prove the safety of the later operation, and hopefully get
an
agreement on the suitability of the change.

I am adding a patch on top of Dhruv's work with that change.

I also implemented the deletion by Buffer (del-buf), about 30% faster than
original deletion by BufferTag (del-tag).

I added the buffer header lock (spin-lock), partition lock (LWLock),
and BufferHashCode to the benchmark. To see when we are entering
the diminishing returns zone.

0003 - 2-step deletion (indistinguishable from 0002), del-buf faster than
del-tag.

op | avg | min | [q1 | median | q3] | p99 | std
-----------+------+------+------+--------+------+------+------
insert | 4.45 | 4.30 | 4.36 | 4.37 | 4.41 | 5.56 | 0.28
hit | 5.96 | 5.81 | 5.86 | 5.88 | 5.93 | 6.82 | 0.27
del-buf | 4.95 | 4.86 | 4.88 | 4.90 | 4.94 | 5.55 | 0.17
insert | 4.46 | 4.39 | 4.41 | 4.42 | 4.45 | 4.86 | 0.10
miss | 5.96 | 5.83 | 5.88 | 5.90 | 5.96 | 6.52 | 0.15
del-tag | 7.32 | 7.14 | 7.18 | 7.20 | 7.28 | 8.49 | 0.61
LWLock-ex | 7.88 | 7.78 | 7.78 | 7.79 | 7.88 | 8.70 | 0.23
LWLock | 7.88 | 7.77 | 7.78 | 7.79 | 7.87 | 8.58 | 0.20
HdrLock | 4.99 | 4.74 | 4.80 | 5.03 | 5.08 | 5.68 | 0.19
hash | 4.44 | 4.37 | 4.38 | 4.39 | 4.44 | 4.97 | 0.16
compare | 1.37 | 1.31 | 1.34 | 1.35 | 1.35 | 1.52 | 0.34
nop | 0.91 | 0.88 | 0.90 | 0.90 | 0.91 | 1.03 | 0.04

0002 - Inline hash (Dhruv's original patch)
op | avg | min | [q1 | median | q3] | p99 | std
-----------+------+------+------+--------+------+------+------
insert | 5.32 | 4.87 | 5.13 | 5.33 | 5.45 | 6.00 | 0.30
hit | 7.15 | 6.53 | 6.92 | 7.16 | 7.32 | 7.77 | 0.30
miss | 7.59 | 6.96 | 7.32 | 7.61 | 7.76 | 8.19 | 0.52
del-tag | 8.59 | 7.85 | 8.30 | 8.60 | 8.78 | 9.32 | 0.50
LWLock-ex | 8.43 | 7.78 | 8.15 | 8.46 | 8.63 | 9.17 | 0.31
LWLock | 8.44 | 7.78 | 8.15 | 8.47 | 8.64 | 9.12 | 0.32
HdrLock | 5.34 | 4.78 | 5.19 | 5.32 | 5.51 | 5.82 | 0.25
hash | 4.74 | 4.38 | 4.59 | 4.75 | 4.85 | 5.17 | 0.20
compare | 1.48 | 1.37 | 1.42 | 1.47 | 1.53 | 1.67 | 0.08
nop | 0.98 | 0.90 | 0.94 | 0.97 | 1.02 | 1.11 | 0.05

0001 - Benchmark (master)
op | avg | min | [q1 | median | q3] | p99 | std
-----------+-------+-------+-------+--------+-------+-------+------
insert | 16.28 | 15.48 | 15.65 | 15.75 | 16.18 | 23.91 | 2.61
hit | 13.31 | 12.51 | 12.72 | 12.76 | 12.99 | 17.45 | 7.70
miss | 13.29 | 12.65 | 12.94 | 12.98 | 13.20 | 16.34 | 2.21
del-tag | 10.31 | 9.66 | 9.96 | 9.99 | 10.15 | 15.33 | 1.72
LWLock-ex | 7.94 | 7.59 | 7.79 | 7.80 | 7.93 | 9.17 | 0.63
LWLock | 7.92 | 7.65 | 7.78 | 7.79 | 7.91 | 10.00 | 0.46
HdrLock | 5.01 | 4.65 | 4.81 | 5.03 | 5.10 | 5.70 | 0.32
hash | 4.56 | 4.39 | 4.47 | 4.51 | 4.57 | 5.18 | 0.20
compare | 1.40 | 1.34 | 1.37 | 1.37 | 1.39 | 1.68 | 0.09
nop | 0.92 | 0.88 | 0.90 | 0.90 | 0.92 | 1.10 | 0.05

Regards,
Alexandre