On 2021/03/11 21:29, Masahiro Ikeda wrote:
On 2021-03-11 11:52, Fujii Masao wrote:
On 2021/03/11 9:38, Masahiro Ikeda wrote:
On 2021-03-10 17:08, Fujii Masao wrote:
On 2021/03/10 14:11, Masahiro Ikeda wrote:
On 2021-03-09 17:51, Fujii Masao wrote:
On 2021/03/05 8:38, Masahiro Ikeda wrote:
On 2021-03-05 01:02, Fujii Masao wrote:
On 2021/03/04 16:14, Masahiro Ikeda wrote:
On 2021-03-03 20:27, Masahiro Ikeda wrote:
On 2021-03-03 16:30, Fujii Masao wrote:
On 2021/03/03 14:33, Masahiro Ikeda wrote:
On 2021-02-24 16:14, Fujii Masao wrote:
On 2021/02/15 11:59, Masahiro Ikeda wrote:
On 2021-02-10 00:51, David G. Johnston wrote:
On Thu, Feb 4, 2021 at 4:45 PM Masahiro Ikeda
<ikeda...@oss.nttdata.com> wrote:
I pgindented the patches.
... <function>XLogWrite</function>, which is invoked during an
<function>XLogFlush</function> request (see ...). This is also
incremented by the WAL receiver during replication.
("which normally called" should be "which is normally called" or
"which normally is called" if you want to keep true to the original)
You missed the adding the space before an opening parenthesis here and
elsewhere (probably copy-paste)
is ether -> is either
"This parameter is off by default as it will repeatedly query the
operating system..."
", because" -> "as"
Thanks, I fixed them.
wal_write_time and the sync items also need the note: "This is also
incremented by the WAL receiver during replication."
I skipped changing it since I separated the stats for the WAL receiver
in pg_stat_wal_receiver.
"The number of times it happened..." -> " (the tally of this event is
reported in wal_buffers_full in....) This is undesirable because ..."
Thanks, I fixed it.
I notice that the patch for WAL receiver doesn't require explicitly
computing the sync statistics but does require computing the write
statistics. This is because of the presence of issue_xlog_fsync but
absence of an equivalent pg_xlog_pwrite. Additionally, I observe that
the XLogWrite code path calls pgstat_report_wait_*() while the WAL
receiver path does not. It seems technically straight-forward to
refactor here to avoid the almost-duplicated logic in the two places,
though I suspect there may be a trade-off for not adding another
function call to the stack given the importance of WAL processing
(though that seems marginalized compared to the cost of actually
writing the WAL). Or, as Fujii noted, go the other way and don't have
any shared code between the two but instead implement the WAL receiver
one to use pg_stat_wal_receiver instead. In either case, this
half-and-half implementation seems undesirable.
OK, as Fujii-san mentioned, I separated the WAL receiver stats.
(v10-0002-Makes-the-wal-receiver-report-WAL-statistics.patch)
Thanks for updating the patches!
I added the infrastructure code to communicate the WAL receiver stats messages
between the WAL receiver and the stats collector, and
the stats for WAL receiver is counted in pg_stat_wal_receiver.
What do you think?
On second thought, this idea seems not good. Because those stats are
collected between multiple walreceivers, but other values in
pg_stat_wal_receiver is only related to the walreceiver process running
at that moment. IOW, it seems strange that some values show dynamic
stats and the others show collected stats, even though they are in
the same view pg_stat_wal_receiver. Thought?
OK, I fixed it.
The stats collected in the WAL receiver is exposed in pg_stat_wal view in v11
patch.
Thanks for updating the patches! I'm now reading 001 patch.
+ /* Check whether the WAL file was synced to disk right now */
+ if (enableFsync &&
+ (sync_method == SYNC_METHOD_FSYNC ||
+ sync_method == SYNC_METHOD_FSYNC_WRITETHROUGH ||
+ sync_method == SYNC_METHOD_FDATASYNC))
+ {
Isn't it better to make issue_xlog_fsync() return immediately
if enableFsync is off, sync_method is open_sync or open_data_sync,
to simplify the code more?
Thanks for the comments.
I added the above code in v12 patch.
+ /*
+ * Send WAL statistics only if WalWriterDelay has elapsed to minimize
+ * the overhead in WAL-writing.
+ */
+ if (rc & WL_TIMEOUT)
+ pgstat_send_wal();
On second thought, this change means that it always takes wal_writer_delay
before walwriter's WAL stats is sent after XLogBackgroundFlush() is called.
For example, if wal_writer_delay is set to several seconds, some values in
pg_stat_wal would be not up-to-date meaninglessly for those seconds.
So I'm thinking to withdraw my previous comment and it's ok to send
the stats every after XLogBackgroundFlush() is called. Thought?
Thanks, I didn't notice that.
Although PGSTAT_STAT_INTERVAL is 500msec, wal_writer_delay's
default value is 200msec and it may be set shorter time.
Yeah, if wal_writer_delay is set to very small value, there is a risk
that the WAL stats are sent too frequently. I agree that's a problem.
Why don't to make another way to check the timestamp?
+ /*
+ * Don't send a message unless it's been at least
PGSTAT_STAT_INTERVAL
+ * msec since we last sent one
+ */
+ now = GetCurrentTimestamp();
+ if (TimestampDifferenceExceeds(last_report, now,
PGSTAT_STAT_INTERVAL))
+ {
+ pgstat_send_wal();
+ last_report = now;
+ }
+
Although I worried that it's better to add the check code in pgstat_send_wal(),
Agreed.
I didn't do so because to avoid to double check PGSTAT_STAT_INTERVAL.
pgstat_send_wal() is invoked pg_report_stat() and it already checks the
PGSTAT_STAT_INTERVAL.
I think that we can do that. What about the attached patch?
Thanks, I thought it's better.
I forgot to remove an unused variable.
The attached v13 patch is fixed.
Thanks for updating the patch!
+ w.wal_write,
+ w.wal_write_time,
+ w.wal_sync,
+ w.wal_sync_time,
It's more natural to put wal_write_time and wal_sync_time next to
each other? That is, what about the following order of columns?
wal_write
wal_sync
wal_write_time
wal_sync_time
Yes, I fixed it.
- case SYNC_METHOD_OPEN:
- case SYNC_METHOD_OPEN_DSYNC:
- /* write synced it already */
- break;
IMO it's better to add Assert(false) here to ensure that we never reach
here, as follows. Thought?
+ case SYNC_METHOD_OPEN:
+ case SYNC_METHOD_OPEN_DSYNC:
+ /* not reachable */
+ Assert(false);
I agree.
Even when a backend exits, it sends the stats via pgstat_beshutdown_hook().
On the other hand, walwriter doesn't do that. Walwriter also should send
the stats even at its exit? Otherwise some stats can fail to be collected.
But ISTM that this issue existed from before, for example checkpointer
doesn't call pgstat_send_bgwriter() at its exit, so it's overkill to fix
this issue in this patch?
Thanks, I thought it's better to do so.
I added the shutdown hook for the walwriter and the checkpointer in v14-0003
patch.
Thanks for 0003 patch!
Isn't it overkill to send the stats in the walwriter-exit-callback? IMO we can
just send the stats only when ShutdownRequestPending is true in the walwriter
main loop (maybe just before calling HandleMainLoopInterrupts()).
If we do this, we cannot send the stats when walwriter throws FATAL error.
But that's ok because FATAL error on walwriter causes the server to crash.
Thought?
Thanks for your comments!
Yes, I agree.
Also ISTM that we don't need to use the callback for that purpose in
checkpointer because of the same reason. That is, we can send the stats
just after calling ShutdownXLOG(0, 0) in HandleCheckpointerInterrupts().
Thought?
Yes, I think so too.
Since ShutdownXLOG() may create restartpoint or checkpoint,
it might generate WAL records.
I'm now not sure how much useful these changes are. As far as I read pgstat.c,
when shutdown is requested, the stats collector seems to exit even when
there are outstanding stats messages. So if checkpointer and walwriter send
the stats in their last cycles, those stats might not be collected.
On the other hand, I can think that sending the stats in the last cycles would
improve the situation a bit than now. So I'm inclined to apply those changes...
I didn't notice that. I agree this is an important aspect.
I understood there is a case that the stats collector exits before the
checkpointer
or the walwriter exits and some stats might not be collected.
IIUC the stats collector basically exits after checkpointer and walwriter exit.
But there seems no guarantee that the stats collector processes
all the messages that other processes have sent during the shutdown of
the server.
Thanks, I understood the above postmaster behaviors.
PMState manages the status and after checkpointer is exited, the postmaster
sends
SIGQUIT signal to the stats collector if the shutdown mode is smart or fast.
(IIUC, although the postmaster kill the walsender, the archiver and
the stats collector at the same time, it's ok because the walsender
and the archiver doesn't send stats to the stats collector now.)
But, there might be a corner case to lose stats sent by background workers like
the checkpointer before they exit (although this is not implemented yet.)
For example,
1. checkpointer send the stats before it exit
2. stats collector receive the signal and break before processing
the stats message from checkpointer. In this case, 1's message is lost.
3. stats collector writes the stats in the statsfiles and exit
Why don't you recheck the coming message is zero just before the 2th procedure?
(v17-0004-guarantee-to-collect-last-stats-messages.patch)
Yes, I was thinking the same. This is the straight-forward fix for this issue.
The stats collector should process all the outstanding messages when
normal shutdown is requested, as the patch does. On the other hand,
if immediate shutdown is requested or emergency bailout (by postmaster death)
is requested, maybe the stats collector should skip those processings
and exit immediately.
But if we implement that, we would need to teach the stats collector
the shutdown type (i.e., normal shutdown or immediate one). Because
currently SIGQUIT is sent to the collector whichever shutdown is requested,
and so the collector cannot distinguish the shutdown type. I'm afraid that
change is a bit overkill for now.
BTW, I found that the collector calls pgstat_write_statsfiles() even at
emergency bailout case, before exiting. It's not necessary to save
the stats to the file in that case because subsequent server startup does
crash recovery and clears that stats file. So it's better to make
the collector exit immediately without calling pgstat_write_statsfiles()
at emergency bailout case? Probably this should be discussed in other
thread because it's different topic from the feature we're discussing here,
though.
IIUC, only the stats collector has another hander for SIGQUIT although
other background processes have a common hander for it and just call _exit(2).
I thought to guarantee when TerminateChildren(SIGTERM) is invoked, don't make
stats
collector shutdown before other background processes are shutdown.
I will make another thread to discuss that the stats collector should know the
shutdown type or not.
If it should be, it's better to make the stats collector exit as soon as
possible if the shutdown type
is an immediate, and avoid losing the remaining stats if it's normal.
+1
I measured the timing of the above in my linux laptop using
v17-measure-timing.patch.
I don't have any strong opinion to handle this case since this result shows to
receive and processes
the messages takes too short time (less than 1ms) although the stats collector
receives the shutdown
signal in 5msec(099->104) after the checkpointer process exits.
Agreed.
```
1615421204.556 [checkpointer] DEBUG: received shutdown request signal
1615421208.099 [checkpointer] DEBUG: proc_exit(-1): 0 callbacks to make
# exit and send the messages
1615421208.099 [stats collector] DEBUG: process BGWRITER stats message
# receive and process the messages
1615421208.099 [stats collector] DEBUG: process WAL stats message
1615421208.104 [postmaster] DEBUG: reaping dead processes
1615421208.104 [stats collector] DEBUG: received shutdown request signal
# receive shutdown request from the postmaster
```
Of course, there is another direction; we can improve the stats collector so
that it guarantees to collect all the sent stats messages. But I'm afraid
this change might be big.
For example, implement to manage background process status in shared memory and
the stats collector collects the stats until another background process exits?
In my understanding, the statistics are not required high accuracy,
it's ok to ignore them if the impact is not big.
If we guarantee high accuracy, another background process like autovacuum
launcher
must send the WAL stats because it accesses the system catalog and might
generate
WAL records due to HOT update even though the possibility is low.
I thought the impact is small because the time uncollected stats are generated
is
short compared to the time from startup. So, it's ok to ignore the remaining
stats
when the process exists.
I agree that it's not worth changing lots of code to collect such stats.
But if we can implement that very simply, isn't it more worth doing
that than current situation because we may be able to collect more
accurate stats.
Yes, I agree.
I attached the patch to send the stats before the wal writer and the
checkpointer exit.
(v17-0001-send-stats-for-walwriter-when-shutdown.patch,
v17-0002-send-stats-for-checkpointer-when-shutdown.patch)
Thanks for making those patches! Firstly I'm reading 0001 and 0002 patches.
Thanks for your comments and for making patches.
Here is the review comments for 0001 patch.
+/* Prototypes for private functions */
+static void HandleWalWriterInterrupts(void);
HandleWalWriterInterrupts() and HandleMainLoopInterrupts() are almost the same.
So I don't think that we need to introduce HandleWalWriterInterrupts(). Instead,
we can just call pgstat_send_wal(true) before HandleMainLoopInterrupts()
if ShutdownRequestPending is true in the main loop. Attached is the patch
I implemented that way. Thought?
I thought there is a corner case that can't send the stats like
You're right! So IMO your patch
(v17-0001-send-stats-for-walwriter-when-shutdown.patch) is better.
```
// First, ShutdownRequstPending = false
if (ShutdownRequestPending) // don't send the stats
pgstat_send_wal(true);
// receive signal and ShutdownRequestPending became true
HandleMainLoopInterrupts(); // proc exit without sending the stats
```
Is it ok because it almost never occurs?
Here is the review comments for 0002 patch.
+static void pgstat_send_checkpointer(void);
I'm inclined to avoid adding the function with the prefix "pgstat_" outside
pgstat.c. Instead, I'm ok to just call both pgstat_send_bgwriter() and
pgstat_report_wal() directly after ShutdownXLOG(). Thought? Patch attached.
Thanks. I agree.
Thanks for the review!
So, barring any objection, I will commit the changes for
walwriter and checkpointer. That is,
v17-0001-send-stats-for-walwriter-when-shutdown.patch
v17-0002-send-stats-for-checkpointer-when-shutdown_fujii.patch
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
