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:
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 researched the past discussion related to writing the stats files when
the immediate
shutdown is requested. And I found the following thread([1]) although
the discussion is
stopped on 12/1/2016.
IIUC, the thread's consensus are
(1) To kill the stats collector soon before writing the stats file is
needed in some case
because there is a possibility that it takes a long time until the
failover happens.
The possible reasons are that disk write speed is slow, stats files
are big, and so on.
(2) It needs to change the behavior from removing all stats files when
the startup does
crash recovery because autovacuum uses the stats.
(3) It's ok that the stats collector exit without calling
pgstat_write_statsfiles() if
the stats file is written every X minutes (using wal or another
mechanism) and startup
process can restore the stats with slightly low freshness.
(4) It needs to find the way how to handle the (2)'s stats file when
deleting on PITR
rewind or stats collector crash happens.
So, I need to ping the threads. But I don't have any idea to handle (4)
yet...
[1]
https://www.postgresql.org/message-id/flat/0A3221C70F24FB45833433255569204D1F5EF25A%40G01JPEXMBYT05
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
I pushed these two patches.
Thanks a lot!
Regards,
--
Masahiro Ikeda
NTT DATA CORPORATION
