On 2020/11/19 16:31, Masahiro Ikeda wrote:
On 2020-11-17 11:46, Fujii Masao wrote:
On 2020/11/16 16:35, Masahiro Ikeda wrote:
On 2020-11-12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
<ikeda...@oss.nttdata.com> wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(&WalStats, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the counters
per process type if necessary. If we adopt this approach, we can easily
extend pg_stat_wal so that any fields can be reported per process type.
I'll remove the above source code because these counters are not useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]-------+------------------------------
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN, auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload generating lots of WAL. If this number is reported high,
to reduce the number of this initialization, we can tune WAL-related parameters
so that more "recycled" WAL files can be held.
3. Number of when WAL is flushed
- wal_write_backend : Total number of WAL data written to the disk by backends
- wal_write_walwriter : Total number of WAL data written to the disk by
walwriter
- wal_sync_backend : Total number of WAL data synced to the disk by backends
- wal_sync_walwriter : Total number of WAL data synced to the disk by walwrite
I think it's useful for tuning "synchronous_commit" and "commit_delay" for
query executions.
If the number of WAL is flushed is high, users can know "synchronous_commit" is
useful for the workload.
I just wonder how useful these counters are. Even without these counters,
we already know synchronous_commit=off is likely to cause the better
performance (but has the risk of data loss). So ISTM that these counters are
not so useful when tuning synchronous_commit.
Thanks, my understanding was wrong.
I agreed that your comments.
I merged the statistics of *_backend and *_walwriter.
I think the sum of them is useful to calculate the average per write/sync time.
For example, per write time is equals wal_write_time / wal_write.
Understood.
Thanks for updating the patch!
Thanks for your comments.
patching file src/include/catalog/pg_proc.dat
Hunk #1 FAILED at 5491.
1 out of 1 hunk FAILED -- saving rejects to file
src/include/catalog/pg_proc.dat.rej
I got this failure when applying the patch. Could you update the patch?
Thanks, I updated the patch.
- Number of times WAL data was written to the disk because WAL
buffers got full
+ Total number of times WAL data written to the disk because WAL
buffers got full
Isn't "was" necessary between "data" and "written"?
Yes, I fixed it.
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>wal_bytes</structfield> <type>bigint</type>
Shouldn't the type of wal_bytes be numeric because the total number of
WAL bytes can exceed the range of bigint? I think that the type of
pg_stat_statements.wal_bytes is also numeric for the same reason.
Thanks, I fixed it.
Since I cast the type of wal_bytes from PgStat_Counter to uint64,
I changed the type of PgStat_MsgWal and PgStat_WalStats too.
+ <entry role="catalog_table_entry"><para role="column_definition">
+ <structfield>wal_write_time</structfield> <type>bigint</type>
Shouldn't the type of wal_xxx_time be double precision,
like pg_stat_database.blk_write_time?
Thanks, I changed it.
Even when fsync is set to off or wal_sync_method is set to open_sync,
wal_sync is incremented. Isn't this behavior confusing?
What do you think about this comment?
Sorry, I'll change to increment wal_sync and wal_sync_time only
if a specific fsync method is called.
I found that we discussed track-WAL-IO-timing feature at the past discussion
about the similar feature [1]. But the feature was droppped from the proposal
patch because there was the performance concern. So probably we need to
revisit the past discussion and benchmark the performance. Thought?
If track-WAL-IO-timing feature may cause performance regression,
it might be an idea to extract wal_records, wal_fpi and wal_bytes parts
from the patch and commit it at first.
[1]
https://postgr.es/m/cajrrpgc6apfugyncpe4qcnxpl8gxkyv1kst+vwjcftcscey...@mail.gmail.com
Thanks, I'll check the thread.
I agree to add basic statistics at first and I attached the patch.
Thanks!
+ /* Send WAL statistics */
+ pgstat_send_wal();
This is not necessary because walwriter generates no WAL data?
+ Total amount of time that has been spent in the portion of
+ WAL data was written to disk by backend and walwriter, in milliseconds
+ (if <xref linkend="guc-track-io-timing"/> is enabled, otherwise zero)
With the patch, track_io_timing controls both IO for data files and
WAL files. But we may want to track only either of them. So it's better
to extend track_io_timing so that we can specify the tracking target
in the parameter? For example, we can make track_io_timing accept
data, wal and all. Or we should introduce new GUC for WAL, e.g.,
track_wal_io_timing? Thought?
OK, I introduced the new GUC "track_wal_io_timing".
I'm afraid that "by backend and walwriter" part can make us thinkg
incorrectly that WAL writes by other processes like autovacuum
are not tracked.
Sorry, I removed "by backend and walwriter".
Thanks for updating the patch!
+WalUsage prevWalUsage;
ISTM that we can declare this as static variable because
it's used only in pgstat.c.
Thanks, I fixed it.
+ memset(&walusage, 0, sizeof(WalUsage));
+ WalUsageAccumDiff(&walusage, &pgWalUsage, &prevWalUsage);
This memset seems unnecessary.
I couldn't understand why this memset is unnecessary.
Since WalUsageAccumDiff not only calculates the difference but also adds the
value,
I thought walusage needs to be initialized.
Yes, you're right! Sorry for noise...
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
