Let PostgreSQL's On Schedule checkpoint write buffer smooth spread cycle by tuning IsCheckpointOnSchedule?

PostgreSQL (<=9.4) trend to smooth buffer write smooth in a
checkpoint_completion_target (checkpoint_timeout or checkpoint_segments),
but when we use synchronous_commit=off, there is a little problem for
the checkpoint_segments
target, because xlog write fast(for full page write which the first page
write after checkpoint), so checkpointer cann't sleep and write buffer not
smooth.
There is an test:
# stap -DMAXSKIPPED=100000 -v 11111 -e '
global s_var, e_var, stat_var;

/* probe smgr__md__read__start(ForkNumber, BlockNumber, Oid, Oid, Oid,
int); */
probe process("/opt/pgsql/bin/postgres").mark("smgr__md__read__start") {
s_var[pid(),1] = gettimeofday_us()
}

/* probe smgr__md__read__done(ForkNumber, BlockNumber, Oid, Oid, Oid, int,
int, int); */
probe process("/opt/pgsql/bin/postgres").mark("smgr__md__read__done") {
e_var[pid(),1] = gettimeofday_us()
if ( s_var[pid(),1] > 0 )
stat_var[pid(),1] <<< e_var[pid(),1] - s_var[pid(),1]
}

/* probe smgr__md__write__start(ForkNumber, BlockNumber, Oid, Oid, Oid,
int); */
probe process("/opt/pgsql/bin/postgres").mark("smgr__md__write__start") {
s_var[pid(),2] = gettimeofday_us()
}

/* probe smgr__md__write__done(ForkNumber, BlockNumber, Oid, Oid, Oid, int,
int, int); */
probe process("/opt/pgsql/bin/postgres").mark("smgr__md__write__done") {
e_var[pid(),2] = gettimeofday_us()
if ( s_var[pid(),2] > 0 )
stat_var[pid(),2] <<< e_var[pid(),2] - s_var[pid(),2]
}

probe process("/opt/pgsql/bin/postgres").mark("buffer__sync__start") {
printf("buffer__sync__start num_buffers: %d, dirty_buffers: %d\n",
$NBuffers, $num_to_write)
}

probe process("/opt/pgsql/bin/postgres").mark("checkpoint__start") {
printf("checkpoint start\n")
}

probe process("/opt/pgsql/bin/postgres").mark("checkpoint__done") {
printf("checkpoint done\n")
}

probe timer.s(1) {
foreach ([v1,v2] in stat_var +) {
if ( @count(stat_var[v1,v2]) >0 ) {
printf("r1_or_w2 %d, pid: %d, min: %d, max: %d, avg: %d, sum: %d,
count: %d\n", v2, v1, @min(stat_var[v1,v2]), @max(stat_var[v1,v2]),
@avg(stat_var[v1,v2]), @sum(stat_var[v1,v2]), @count(stat_var[v1,v2]))
}
}

printf("----------------------------------end-----------------------------\n")
delete s_var
delete e_var
delete stat_var
}'

Use the test table and data:
create table tbl(id primary key,info text,crt_time timestamp);
insert into tbl select generate_series(1,50000000),now(),now();

Use pgbench test it.
$ vi test.sql
\setrandom id 1 50000000
update tbl set crt_time=now() where id = :id ;

$ pgbench -M prepared -n -r -f ./test.sql -P 1 -c 28 -j 28 -T 100000000
When on schedule checkpoint occure , the tps:
progress: 255.0 s, 58152.2 tps, lat 0.462 ms stddev 0.504
progress: 256.0 s, 31382.8 tps, lat 0.844 ms stddev 2.331
progress: 257.0 s, 14615.5 tps, lat 1.863 ms stddev 4.554
progress: 258.0 s, 16258.4 tps, lat 1.652 ms stddev 4.139
progress: 259.0 s, 17814.7 tps, lat 1.526 ms stddev 4.035
progress: 260.0 s, 14573.8 tps, lat 1.825 ms stddev 5.592
progress: 261.0 s, 16736.6 tps, lat 1.600 ms stddev 5.018
progress: 262.0 s, 19060.5 tps, lat 1.448 ms stddev 4.818
progress: 263.0 s, 20553.2 tps, lat 1.290 ms stddev 4.146
progress: 264.0 s, 26223.0 tps, lat 1.042 ms stddev 3.711
progress: 265.0 s, 31953.0 tps, lat 0.836 ms stddev 2.837
progress: 266.0 s, 43396.1 tps, lat 0.627 ms stddev 1.615
progress: 267.0 s, 50487.8 tps, lat 0.533 ms stddev 0.647
progress: 268.0 s, 53537.7 tps, lat 0.502 ms stddev 0.598
progress: 269.0 s, 54259.3 tps, lat 0.496 ms stddev 0.624
progress: 270.0 s, 56139.8 tps, lat 0.479 ms stddev 0.524

The parameters for onschedule checkpoint:
checkpoint_segments = 512
checkpoint_timeout = 5min
checkpoint_completion_target = 0.9

stap's output :
there is 156467 dirty blocks, we can see the buffer write per second, write
buffer is not smooth between time target.
but between xlog target.
156467/(4.5*60*0.9) = 579.5 write per second.

checkpoint start
buffer__sync__start num_buffers: 262144, dirty_buffers: 156467
r1_or_w2 2, pid: 19848, min: 41, max: 1471, avg: 49, sum: 425291, count:
8596
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 41, max: 153, avg: 49, sum: 450597, count: 9078
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 41, max: 643, avg: 51, sum: 429193, count: 8397
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 41, max: 1042, avg: 55, sum: 449091, count:
8097
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 41, max: 254, avg: 52, sum: 296668, count: 5617
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 39, max: 171, avg: 54, sum: 321027, count: 5851
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 41, max: 138, avg: 60, sum: 300056, count: 4953
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 42, max: 1217, avg: 65, sum: 312859, count:
4748
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 41, max: 1371, avg: 56, sum: 353905, count:
6304
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 41, max: 358, avg: 58, sum: 236254, count: 4038
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 34, max: 1239, avg: 63, sum: 296906, count:
4703
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 31, max: 17408, avg: 63, sum: 415234, count:
6534
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 31, max: 5486, avg: 57, sum: 190345, count:
3318
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 29, max: 510, avg: 53, sum: 136221, count: 2563
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 32, max: 733, avg: 52, sum: 108327, count: 2070
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 34, max: 382, avg: 53, sum: 96157, count: 1812
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 43, max: 327, avg: 53, sum: 83641, count: 1571
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 33, max: 102, avg: 54, sum: 79991, count: 1468
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 35, max: 88, avg: 53, sum: 74338, count: 1389
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 32, max: 86, avg: 52, sum: 65710, count: 1243
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 30, max: 347, avg: 52, sum: 66866, count: 1263
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 31, max: 93, avg: 54, sum: 75642, count: 1398
----------------------------------end-----------------------------
r1_or_w2 2, pid: 19848, min: 33, max: 100, avg: 51, sum: 62302, count: 1216
......

I think we can add an condition to the IsCheckpointOnSchedule,
if (synchronous_commit != SYNCHRONOUS_COMMIT_OFF)
{
recptr = GetInsertRecPtr();
elapsed_xlogs = (((double) (recptr -
ckpt_start_recptr)) / XLogSegSize) / CheckPointSegments;

if (progress < elapsed_xlogs)
{
ckpt_cached_elapsed = elapsed_xlogs;
return false;
}
}

# vi src/backend/postmaster/checkpointer.c

#include "access/xact.h"

/*
* IsCheckpointOnSchedule -- are we on schedule to finish this checkpoint
* in time?
*
* Compares the current progress against the time/segments elapsed since
last
* checkpoint, and returns true if the progress we've made this far is
greater
* than the elapsed time/segments.
*/
static bool
IsCheckpointOnSchedule(double progress)
{
XLogRecPtr recptr;
struct timeval now;
double elapsed_xlogs,
elapsed_time;

Assert(ckpt_active);

/* Scale progress according to checkpoint_completion_target. */
progress *= CheckPointCompletionTarget;

/*
* Check against the cached value first. Only do the more expensive
* calculations once we reach the target previously calculated.
Since
* neither time or WAL insert pointer moves backwards, a freshly
* calculated value can only be greater than or equal to the cached
value.
*/
if (progress < ckpt_cached_elapsed)
return false;

/*
* Check progress against WAL segments written and
checkpoint_segments.
*
* We compare the current WAL insert location against the location
* computed before calling CreateCheckPoint. The code in XLogInsert
that
* actually triggers a checkpoint when checkpoint_segments is
exceeded
* compares against RedoRecptr, so this is not completely accurate.
* However, it's good enough for our purposes, we're only
calculating an
* estimate anyway.
*/
if (!RecoveryInProgress())
{
if (synchronous_commit != SYNCHRONOUS_COMMIT_OFF)
{
recptr = GetInsertRecPtr();
elapsed_xlogs = (((double) (recptr -
ckpt_start_recptr)) / XLogSegSize) / CheckPointSegments;

if (progress < elapsed_xlogs)
{
ckpt_cached_elapsed = elapsed_xlogs;
return false;
}
}
}

/*
* Check progress against time elapsed and checkpoint_timeout.
*/
gettimeofday(&now, NULL);
elapsed_time = ((double) ((pg_time_t) now.tv_sec - ckpt_start_time)
+
now.tv_usec / 1000000.0) /
CheckPointTimeout;

if (progress < elapsed_time)
{
ckpt_cached_elapsed = elapsed_time;
return false;
}

/* It looks like we're on schedule. */
return true;
}

# gmake && gmake install

$ pg_ctl restart -m fast
Test again:
progress: 291.0 s, 63144.9 tps, lat 0.426 ms stddev 0.383
progress: 292.0 s, 55063.7 tps, lat 0.480 ms stddev 1.433
progress: 293.0 s, 12225.3 tps, lat 2.238 ms stddev 4.460
progress: 294.0 s, 16436.4 tps, lat 1.621 ms stddev 4.043
progress: 295.0 s, 18516.5 tps, lat 1.444 ms stddev 3.286
progress: 296.0 s, 21983.7 tps, lat 1.251 ms stddev 2.941
progress: 297.0 s, 25759.7 tps, lat 1.034 ms stddev 2.356
progress: 298.0 s, 33139.4 tps, lat 0.821 ms stddev 1.676
progress: 299.0 s, 41904.9 tps, lat 0.644 ms stddev 1.134
progress: 300.0 s, 52432.9 tps, lat 0.513 ms stddev 0.470
progress: 301.0 s, 57115.4 tps, lat 0.471 ms stddev 0.325
progress: 302.0 s, 59422.1 tps, lat 0.452 ms stddev 0.297
progress: 303.0 s, 59860.5 tps, lat 0.449 ms stddev 0.309

We can see checkpointer wiriter buffer smooth(spread time perid) this time.
checkpoint start
----------------------------------end-----------------------------
buffer__sync__start num_buffers: 262144, dirty_buffers: 156761
r1_or_w2 2, pid: 22334, min: 51, max: 137, avg: 60, sum: 52016, count: 860
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 51, max: 108, avg: 58, sum: 35526, count: 604
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 51, max: 145, avg: 71, sum: 39779, count: 559
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 52, max: 172, avg: 79, sum: 47279, count: 594
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 44, max: 160, avg: 63, sum: 36907, count: 581
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 51, max: 113, avg: 61, sum: 33895, count: 552
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 51, max: 116, avg: 61, sum: 38177, count: 617
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 51, max: 113, avg: 62, sum: 34199, count: 550
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 53, max: 109, avg: 65, sum: 39842, count: 606
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 50, max: 118, avg: 64, sum: 35099, count: 545
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 50, max: 107, avg: 64, sum: 39027, count: 606
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 51, max: 114, avg: 62, sum: 34054, count: 545
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 47, max: 106, avg: 63, sum: 38573, count: 605
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 48, max: 101, avg: 62, sum: 38051, count: 607
----------------------------------end-----------------------------
r1_or_w2 2, pid: 22334, min: 42, max: 103, avg: 61, sum: 33596, count: 545

But there is also a little problem, When PostgreSQL write xlog reach
checkpoint_segments earlier then checkpoint_timeout, the next checkpoint
will start soon, so we must tuning the checkpoint_segments larger when the
checkpoint occure busy.

Regards,
Digoal

--
公益是一辈子的事,I'm Digoal,Just Do It.