Обсуждение: Optimize commit performance with a large number of 'on commit delete rows' temp tables

On Jul 6, 2024, at 00:15, Tom Lane <tgl@sss.pgh.pa.us> wrote:

<feichanghong@qq.com> writes:

PostgreSQL maintains a list of temporary tables for 'on commit
drop/delete rows' via an on_commits list in the session. Once a
transaction accesses a temp table or namespace, the
XACT_FLAGS_ACCESSEDTEMPNAMESPACE flag is set. Before committing, the
PreCommit_on_commit_actions function truncates all 'commit delete
rows' temp tables, even those not accessed in the current transaction.
Commit performance can degrade if there are many such temp tables.

Hmm.  I can sympathize with wanting to improve the performance of
this edge case, but it is an edge case: you are the first to
complain about it.  You cannot trash the performance of more typical
cases in order to get there ...

In the attached patch (based on HEAD):
- A Bloom filter (can also be a list or hash table) maintains
the temp tables accessed by the current transaction.

... and I'm afraid this proposal may do exactly that.  Our bloom
filters are pretty heavyweight objects, so making one in situations
where it buys nothing is likely to add a decent amount of overhead.
(I've not tried to quantify that for this particular patch.)

I wonder if we could instead add marker fields to the OnCommitItem
structs indicating whether their rels were touched in the current
transaction, and use those to decide whether we need to truncate.

Another possibility is to make the bloom filter only when the
number of OnCommitItems exceeds some threshold (compare d365ae705).

BTW, I wonder if we could improve PreCommit_on_commit_actions by
having it just quit immediately if XACT_FLAGS_ACCESSEDTEMPNAMESPACE
is not set.  I think that must be set if any ON COMMIT DROP tables
have been made, so there should be nothing to do if not.  In normal
cases that's not going to buy much because the OnCommitItems list
is short, but in your scenario maybe it could win.

On Jul 8, 2024, at 12:18, wenhui qiu <qiuwenhuifx@gmail.com> wrote:

Hi feichanghong
    I don't think it's acceptable to introduce a patch to fix a problem that leads to performance degradation, or can we take tom's suggestion to optimise PreCommit_on_commit_actions?  I think it to miss the forest for the trees

You're right, any performance regression is certainly unacceptable. That's why

we've introduced a threshold. The bloom filter optimization is only applied

when the number of temporary tables exceeds this threshold. Test data also

reveals that with a threshold of 10, barring cases where all temporary tables

are implicated in a transaction, there's hardly any performance loss.

"Improve PreCommit_on_commit_actions by having it just quit immediately if

XACT_FLAGS_ACCESSEDTEMPNAMESPACE is not set" can only reduce the overhead of

traversing the OnCommitItem List but still doesn't address the issue with

temporary table truncation.

Looking forward to more suggestions!

Hi wenhui,

I carefully analyzed the reason for the performance regression with fewer

temporary tables in the previous patch (v1-0002-): the k_hash_funcs determined

by the bloom_create function were 10(MAX_HASH_FUNCS), which led to an excessive

calculation overhead for the bloom filter.

Based on the calculation formula for the bloom filter, when the number of items

is 100 and k_hash_funcs is 2, the false positive rate for a 1KB bloom filter is

0.0006096; when the number of items is 1000, the false positive rate is

0.048929094. Therefore, k_hash_funcs of 2 can already achieve a decent false

positive rate, while effectively reducing the computational overhead of the

bloom filter.

I have re-implemented a bloom_create_v2 function to create a bloom filter with

a specified number of hash functions and specified memory size.

From the test data below, it can be seen that the new patch in the attachment

(v1-0003-) does not lead to performance regression in any scenario.

Furthermore, the default threshold value can be lowered to 2.

Here is the TPS performance data for different numbers of temporary tables

under different thresholds, as compared with the head (98347b5a). The testing

tool used is pgbench, with the workload being to insert into one temporary

table (when the number of temporary tables is 0, the workload is SELECT 1):

|tablenum      |0           |1           |2          |5          |10         |100        |1000       |

|--------------|------------|------------|-----------|-----------|-----------|-----------|-----------|

|head(98347b5a)|39912.722209|10064.306268|9183.871298|7452.071689|5641.487369|1073.203851|114.530958 |

|threshold-2   |40097.047974|10009.598155|9982.172866|9955.651235|9999.338901|9785.626296|8278.828828|

Here is the TPS performance data for different numbers of temporary tables

at a threshold of 2, compared with the head (commit 98347b5a). The testing tool

is pgbench, with the workload being to insert into all temporary tables:

|table num     |1          |2          | 5         |10         |100       |1000     |

|--------------|-----------|-----------|-----------|-----------|----------|---------|

|head(98347b5a)|7243.945042|5734.545012|3627.290594|2262.594766|297.856756|27.745808|

|threshold-2   |7289.171381|5740.849676|3626.135510|2207.439931|293.145036|27.020953|

I have previously researched the implementation of the Global Temp Table (GTT)

you mentioned, and it have been used in Alibaba Cloud's PolarDB (Link [1]).

GTT can prevent truncation operations on temporary tables that have not been

accessed by the current session (those not in the OnCommitItem List), but GTT

that have been accessed by the current session still need to be truncated at

commit time.Therefore, GTT also require the optimizations mentioned in the

above patch.

[1] https://www.alibabacloud.com/help/en/polardb/polardb-for-oracle/using-global-temporary-tables?spm=a3c0i.23458820.2359477120.1.66e16e9bUpV7cK