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.
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
```
// 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.
BTW, I found BgWriterStats.m_timed_checkpoints is not counted in
ShutdownLOG()
and we need to count it if to collect stats before it exits.
Maybe m_requested_checkpoints should be incremented in that case?
I thought this should be incremented
because it invokes the methods with CHECKPOINT_IS_SHUTDOWN.
Yes.
OK, thanks.
```ShutdownXLOG()
CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
```
I fixed in v17-0002-send-stats-for-checkpointer-when-shutdown.patch.
In addition, I rebased the patch for WAL receiver.
(v17-0003-Makes-the-wal-receiver-report-WAL-statistics.patch)
Thanks! Will review this later.
Thanks a lot!
Regards,
--
Masahiro Ikeda
NTT DATA CORPORATION
