22.01.2025 09:09, Japin Li пишет:
On Sun, 19 Jan 2025 at 17:56, Yura Sokolov <y.soko...@postgrespro.ru> wrote:
17.01.2025 17:00, Zhou, Zhiguo пишет:
On 1/16/2025 10:00 PM, Yura Sokolov wrote:
Good day, Zhiguo.
Excuse me, I feel sneaky a bit, but I've started another thread
just about increase of NUM_XLOGINSERT_LOCK, because I can measure
its effect even on my working notebook (it is another one: Ryzen
5825U limited to @2GHz).
http://postgr.es/m/flat/3b11fdc2-9793-403d-
b3d4-67ff9a00d447%40postgrespro.ru
-----
regards
Yura Sokolov aka funny-falcon
Good day, Yura!
Thank you for keeping me informed. I appreciate your proactive
approach and understand the importance of exploring different angles
for optimization. Your patch is indeed fundamental to our ongoing
work on the lock-free xlog reservation, and I'm eager to see how it
can further enhance our efforts.
I will proceed to test the performance impact of your latest patch
when combined with the lock-free xlog reservation patch. This will
help us determine if there's potential for additional
optimization. Concurrently, with your permission, I'll try to refine
the hash-table- based implementation for your further review. WDYT?
Good day, Zhiguo
Here's version of "hash-table reservation" with both 32bit and 64bit
operations (depending on PG_HAVE_ATOMIC_U64_SIMULATION, or may be
switched by hand).
64bit version uses other protocol with a bit lesser atomic
operations. I suppose it could be a bit faster. But I can't prove it
now.
btw, you wrote:
Major issue:
- `SetPrevRecPtr` and `GetPrevRecPtr` do non-atomic write/read
with on
platforms where MAXALIGN != 8 or without native 64
load/store. Branch
with 'memcpy` is rather obvious, but even pointer de-referencing on
"lucky case" is not safe either.
I have no idea how to fix it at the moment.
Indeed, non-atomic write/read operations can lead to safety issues in
some situations. My initial thought is to define a bit near the
prev-link to flag the completion of the update. In this way, we could
allow non-atomic or even discontinuous write/read operations on the
prev-link, while simultaneously guaranteeing its atomicity through
atomic operations (as well as memory barriers) on the flag bit. What
do you think of this as a viable solution?
There is a way to order operations:
- since SetPrevRecPtr stores start of record as LSN, its lower 32bits
are certainly non-zero (record could not start at the beginning of a
page).
- so SetPrevRecPtr should write high 32bits, issue write barrier, and
then write lower 32bits,
- and then GetPrevRecPtr should first read lower 32bits, and if it is
not zero, then issue read barrier and read upper 32bits.
This way you will always read correct prev-rec-ptr on platform without
64bit atomics. (because MAXALING >= 4 and PostgreSQL requires 4 byte
atomicity for several years).
Hi, Yura Sokolov
Thanks for updating the patch.
I test the v2 patch using BenchmarkSQL 1000 warehouse, and here is the tpmC
result:
case | min | avg | max
--------------------+------------+------------+--------------
master (patched) | 988,461.89 | 994,916.50 | 1,000,362.40
master (44b61efb79) | 857,028.07 | 863,174.59 | 873,856.92
The patch provides a significant improvement.
I just looked through the patch, here are some comments.
1.
The v2 patch can't be applied cleanly.
Applying: Lock-free XLog Reservation using lock-free hash-table
.git/rebase-apply/patch:33: trailing whitespace.
.git/rebase-apply/patch:37: space before tab in indent.
{
.git/rebase-apply/patch:38: space before tab in indent.
int i;
.git/rebase-apply/patch:39: trailing whitespace.
.git/rebase-apply/patch:46: space before tab in indent.
LWLockReleaseClearVar(&WALInsertLocks[i].l.lock,
warning: squelched 4 whitespace errors
warning: 9 lines add whitespace errors.
2.
And there is a typo:
+ * PrevLinksHash is a lock-free hash table based on Cuckoo algorith. It is
+ * mostly 4 way: for every element computed two positions h1, h2, and
s/algorith/algorithm/g
Hi, Japin
Thank you a lot for measuring and comments.
May I ask you to compare not only against master, but against straight
increase of NUM_XLOGINSERT_LOCKS to 128 as well?
This way the profit from added complexity will be more obvious: does it
pay for self or not.
-------
regards
Yura
