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Lift limitation that PGPROC->links must be the first field
- 98347b5a3ab1 18.0 cited
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Optimize commit performance with a large number of 'on commit delete rows' temp tables
feichanghong <feichanghong@qq.com> — 2024-07-05T15:19:22Z
Hi hackers, # Background 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. In practice, users created many 'commit delete rows' temp tables in a session, but each transaction only accessed a few. With varied access frequency, users were reluctant to change to 'on commit drop'. Below is an example showing the effect of the number of temp tables on commit performance: ``` -- 100 DO $$ DECLARE begin FOR i IN 1..100 LOOP EXECUTE format('CREATE TEMP TABLE temp_table_%s (id int) on commit delete ROWS', i) ; END LOOP; END; $$; postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 1.325 ms postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 1.330 ms ``` ``` -- 1000 DO $$ DECLARE begin FOR i IN 1..1000 LOOP EXECUTE format('CREATE TEMP TABLE temp_table_%s (id int) on commit delete ROWS', i) ; END LOOP; END; $$; postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 10.939 ms postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 10.955 ms ``` ``` -- 10000 DO $$ DECLARE begin FOR i IN 1..10000 LOOP EXECUTE format('CREATE TEMP TABLE temp_table_%s (id int) on commit delete ROWS', i) ; END LOOP; END; $$; postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 110.253 ms postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 175.875 ms ``` # Solution An intuitive solution is to truncate only the temp tables that the current process has accessed upon transaction commit. 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. - Only temp tables filtered through the Bloom filter need truncation. False positives may occur, but they are acceptable. - The Bloom filter is reset at the start of the transaction, indicating no temp tables have been accessed by the current transaction yet. After optimization, the performance for the same case is as follows: ``` -- 100 DO $$ DECLARE begin FOR i IN 1..100 LOOP EXECUTE format('CREATE TEMP TABLE temp_table_%s (id int) on commit delete ROWS', i) ; END LOOP; END; $$; postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 0.447 ms postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 0.453 ms ``` ``` -- 1000 DO $$ DECLARE begin FOR i IN 1..1000 LOOP EXECUTE format('CREATE TEMP TABLE temp_table_%s (id int) on commit delete ROWS', i) ; END LOOP; END; $$; postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 0.531 ms postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 0.567 ms ``` ``` -- 10000 DO $$ DECLARE begin FOR i IN 1..10000 LOOP EXECUTE format('CREATE TEMP TABLE temp_table_%s (id int) on commit delete ROWS', i) ; END LOOP; END; $$; postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 1.370 ms postgres=# insert into temp_table_1 select 1; INSERT 0 1 Time: 1.362 ms ``` Hoping for some suggestions from hackers. Best Regards, Fei Changhong -
Re: Optimize commit performance with a large number of 'on commit delete rows' temp tables
Tom Lane <tgl@sss.pgh.pa.us> — 2024-07-05T16:15:03Z
"=?ISO-8859-1?B?ZmVpY2hhbmdob25n?=" <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. regards, tom lane
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Re: Optimize commit performance with a large number of 'on commit delete rows' temp tables
feichanghong <feichanghong@qq.com> — 2024-07-05T18:08:30Z
Thank you for your attention and suggestions. > 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.) Yes, this is an edge case, but we have more than one customer facing the issue, and unfortunately, they are not willing to modify their service code. We should indeed avoid negatively impacting typical cases: - Each connection requires an extra 1KB for the filter (the original bloom filter implementation had a minimum of 1MB, which I've adjusted to this smaller value). - The filter is reset at the start of each transaction, which is unnecessary for sessions that do not access temporary tables. - In the PreCommit_on_commit_actions function, each 'on commit delete rows' temporary table has to be filtered through the bloom filter, which incurs some CPU overhead. However, this might be negligible compared to the IO cost of truncation. Adding a threshold for using the bloom filter is a good idea. We can create the bloom filter only when the current number of OnCommitItems exceeds the threshold at the start of a transaction, which should effectively avoid affecting typical cases. I will provide a new patch later to implement this. > 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. Adding a flag to OnCommitItem to indicate whether the temp table was accessed by the current transaction is feasible. But, locating the OnCommitItem by relid efficiently when opening a relation may require an extra hash table to map relids to OnCommitItems. > 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. I also think when XACT_FLAGS_ACCESSEDTEMPNAMESPACE is not set, it's unnecessary to iterate over on_commits (unless I'm overlooking something), which would be beneficial for the aforementioned scenarios as well. Best Regards, Fei Changhong
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Re: Optimize commit performance with a large number of 'on commit delete rows' temp tables
feichanghong <feichanghong@qq.com> — 2024-07-05T19:39:41Z
The patch in the attachment, compared to the previous one, adds a threshold for using the bloom filter. The current ON_COMMITS_FILTER_THRESHOLD is set to 64, which may not be the optimal value. Perhaps this threshold could be configured as a GUC parameter? Best Regards, Fei Changhong
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Re: Optimize commit performance with a large number of 'on commit delete rows' temp tables
wenhui qiu <qiuwenhuifx@gmail.com> — 2024-07-07T13:32:24Z
Hi feichanghong Thanks for updating the patch ,I think could be configured as a GUC parameter,PostgreSQL has too many static variables that are written to death and explicitly stated in the code comments may later be designed as parameters. Now that more and more applications that previously used oracle are migrating to postgresql, there will be more and more scenarios where temporary tables are heavily used.Because oracle will global temporary tablespace optimised for this business scenario, which works well in oracle, migrating to pg faces very tricky performance issues,I'm sure the patch has vaule Best Regards feichanghong <feichanghong@qq.com> 于2024年7月6日周六 03:40写道： > The patch in the attachment, compared to the previous one, adds a > threshold for > using the bloom filter. The current ON_COMMITS_FILTER_THRESHOLD is set to > 64, > which may not be the optimal value. Perhaps this threshold could be > configured > as a GUC parameter? > ------------------------------ > Best Regards, > Fei Changhong > -
Re: Optimize commit performance with a large number of 'on commit delete rows' temp tables
feichanghong <feichanghong@qq.com> — 2024-07-08T02:35:44Z
Hi wenhui, Thank you for your suggestions. I have supplemented some performance tests. 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): | table num | 0 | 1 | 5 | 10 | 100 | 1000 | |---------------|--------------|--------------|-------------|-------------|-------------|-------------| | head 98347b5a | 39912.722209 | 10064.306268 | 7452.071689 | 5641.487369 | 1073.203851 | 114.530958 | | threshold 1 | 40332.367414 | 7078.117192 | 7044.951156 | 7020.249434 | 6893.652062 | 5826.597260 | | threshold 5 | 40173.562744 | 10017.532933 | 7023.770203 | 7024.283577 | 6919.769315 | 5806.314494 | Here is the TPS performance data for different numbers of temporary tables at a threshold of 5, compared with the head (commit 98347b5a). The testing tool is pgbench, with the workload being to insert into all temporary tables: | table num | 1 | 5 | 10 | 100 | 1000 | |---------------|-------------|-------------|-------------|------------|-----------| | head 98347b5a | 7243.945042 | 3627.290594 | 2262.594766 | 297.856756 | 27.745808 | | threshold 5 | 7287.764656 | 3130.814888 | 2038.308763 | 288.226032 | 27.705149 | According to test results, the patch does cause some performance loss with fewer temporary tables, but benefits are substantial when many temporary tables are used. The specific threshold could be set to 10 (HDDs may require a smaller one). I've provided two patches in the attachments, both with a default threshold of 10. One has the threshold configured as a GUC parameter, while the other is hardcoded to 10. Best Regards, Fei Changhong
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Re: Optimize commit performance with a large number of 'on commit delete rows' temp tables
wenhui qiu <qiuwenhuifx@gmail.com> — 2024-07-08T04:18:17Z
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 Best Regards, feichanghong <feichanghong@qq.com> 于2024年7月8日周一 10:35写道： > Hi wenhui, > > Thank you for your suggestions. I have supplemented some performance tests. > > 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): > > | table num | 0 | 1 | 5 | 10 > | 100 | 1000 | > > |---------------|--------------|--------------|-------------|-------------|-------------|-------------| > | head 98347b5a | 39912.722209 | 10064.306268 | 7452.071689 | 5641.487369 > | 1073.203851 | 114.530958 | > | threshold 1 | 40332.367414 | 7078.117192 | 7044.951156 | 7020.249434 > | 6893.652062 | 5826.597260 | > | threshold 5 | 40173.562744 | 10017.532933 | 7023.770203 | 7024.283577 > | 6919.769315 | 5806.314494 | > > Here is the TPS performance data for different numbers of temporary tables > at a threshold of 5, compared with the head (commit 98347b5a). The testing > tool > is pgbench, with the workload being to insert into all temporary tables: > > | table num | 1 | 5 | 10 | 100 | > 1000 | > > |---------------|-------------|-------------|-------------|------------|-----------| > | head 98347b5a | 7243.945042 | 3627.290594 | 2262.594766 | 297.856756 | > 27.745808 | > | threshold 5 | 7287.764656 | 3130.814888 | 2038.308763 | 288.226032 | > 27.705149 | > > According to test results, the patch does cause some performance loss with > fewer temporary tables, but benefits are substantial when many temporary > tables > are used. The specific threshold could be set to 10 (HDDs may require a > smaller > one). > > I've provided two patches in the attachments, both with a default > threshold of 10. > One has the threshold configured as a GUC parameter, while the other is > hardcoded > to 10. > ------------------------------ > Best Regards, > Fei Changhong > -
Re: Optimize commit performance with a large number of 'on commit delete rows' temp tables
feichanghong <feichanghong@qq.com> — 2024-07-08T04:41:43Z
Hi wenhui, > 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! Best Regards, Fei Changhong
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Re: Optimize commit performance with a large number of 'on commit delete rows' temp tables
wenhui qiu <qiuwenhuifx@gmail.com> — 2024-07-08T06:05:09Z
Hi feichanghong I think adding an intercept this way is better than implementing a global temp table,there is a path to implement a global temporary table ( https://www.postgresql.org/message-id/1A1A6EDC-D0EC-47B0-BD21-C2ACBAEA65E4@alibaba-inc.com),you can consult with them ,they work at Alibaba Best Regards, feichanghong <feichanghong@qq.com> 于2024年7月8日周一 12:42写道： > Hi wenhui, > > 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! > > Best Regards, > Fei Changhong > > -
Re: Optimize commit performance with a large number of 'on commit delete rows' temp tables
feichanghong <feichanghong@qq.com> — 2024-07-08T14:17:09Z
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 Best Regards, Fei Changhong