A Deep Dive Into PostgreSQL Vacuum Monitoring With BPFtrace

A Deep Dive Into PostgreSQL Vacuum Monitoring With BPFtrace

Observability enables engineers to quickly troubleshoot and fix issues in production. For those using the Linux kernel, the extended Berkeley Packet Filter (eBPF) technology allows application monitoring with minimal overhead. For example, you can use UProbes to trace the invocation and exit of functions in programs.

Modern database observability tools (e.g., funccount) are built on top of eBPF. However, these fully flagged tools are often written in C and Python and require some development effort when a "quick and dirty" solution for a particular observation is sometimes more than sufficient. However, with BPFtrace, a high-level tracing language for Linux eBPF, users can create eBPF programs with only a few lines of code.

In this tutorial, we develop a simple BPFtrace program to observe the execution of vacuum calls in PostgreSQL and measure and print their execution times.

The Environment

PostgreSQL uses vacuum operations to reclaim space from dead (e.g., updated or deleted) tuples. For more information on how this process works and why it is an important one to monitor, read this article.

In this tutorial, we will trace the vacuum calls and determine the required time per table for the vacuum operations. We will use a PostgreSQL 14 server, with the binary is located at /home/jan/postgresqlsandbox/bin/REL_14_2_DEBUG/bin/postgres.

In addition, the examples are executed in a database with these two tables:

CREATE TABLE testtable1 (
   id int NOT NULL,
   value int NOT NULL
);

CREATE TABLE testtable2 (
   id int NOT NULL,
   value int NOT NULL
);
Note: Depending on the used C compiler and applied optimizations, the symbols of internal functions (i.e., as static declared) may not be visible. In this case, you can not use UProbes to trace the function invocations. There are two possible solutions to address this issue: (1) remove the static modifier from the function declaration and recompile PostgreSQL, or (2) create a complete debug build of PostgreSQL.


Tracing PostgreSQL Vacuum Operations Using funclatency

Let’s explore the existing solutions before developing our tool to trace the vacuum operations.

The tool funclatencyis available for most Linux distributions (on Debian, it’s part of the package bpfcc-tools and renamed to funclatency-bpfcc) and allows it to trace a function enter and exit and measure the function latency (i.e., the time it takes a function to complete).

In PostgreSQL, the function vacuum_rel is invoked when a vacuum operation on a relation is performed. To trace these function calls with funclatency-bpfcc, you need to provide the path of the PostgreSQL binary and the function name. For instance:‌

$ sudo funclatency-bpfcc -r /home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel

Tracing 1 functions for "/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel"... Hit Ctrl-C to end.

Afterward, an eBPF program is loaded into the Linux kernel, and a UProbe is defined and observes the beginning of the function call while another UProbe observes its exit. The latency between these two events is measured and stored.

To execute some vacuum operations, we perform the following SQL statement in a second session:

database=# VACUUM FULL;
VACUUM FULL


This SQL statement triggers PostgreSQL to perform a vacuum operation of all tables of the current database—this takes some time. After the vacuum operations are done, the funclatency-bpfcc program can be stopped (by executing CTRL+C), ending the observation of the binary and showing the recorded execution times on the terminal.

$ sudo funclatency-bpfcc -r /home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel
[...]
^C
Function = b'vacuum_rel' [876997]
     nsecs               : count     distribution
         0 -> 1          : 0        |                                        |
         2 -> 3          : 0        |                                        |
         4 -> 7          : 0        |                                        |
         8 -> 15         : 0        |                                        |
        16 -> 31         : 0        |                                        |
        32 -> 63         : 0        |                                        |
        64 -> 127        : 0        |                                        |
       128 -> 255        : 0        |                                        |
       256 -> 511        : 0        |                                        |
       512 -> 1023       : 0        |                                        |
      1024 -> 2047       : 0        |                                        |
      2048 -> 4095       : 0        |                                        |
      4096 -> 8191       : 0        |                                        |
      8192 -> 16383      : 0        |                                        |
     16384 -> 32767      : 0        |                                        |
     32768 -> 65535      : 0        |                                        |
     65536 -> 131071     : 0        |                                        |
    131072 -> 262143     : 0        |                                        |
    262144 -> 524287     : 0        |                                        |
    524288 -> 1048575    : 0        |                                        |
   1048576 -> 2097151    : 0        |                                        |
   2097152 -> 4194303    : 0        |                                        |
   4194304 -> 8388607    : 2        |*                                       |
   8388608 -> 16777215   : 13       |***********                             |
  16777216 -> 33554431   : 44       |****************************************|
  33554432 -> 67108863   : 7        |******                                  |
  67108864 -> 134217727  : 1        |                                        |

avg = 22765358 nsecs, total: 1525279002 nsecs, count: 67

Detaching...

The output contains the information that the function vacuum_rel was called 67 times, and the average function time is 22765358 nsecs.

In addition, a histogram of the function latency is printed, providing a lot of helpful information. However, while we now know the number and duration of vacuum calls, we still don’t know the duration of the individual vacuum calls for each relation.

This is something that this tool does not support because it does not evaluate the function parameters (e.g., the Oid of relation that the current function invocation should vacuum). But this is something we can do with bpftrace.

Using BPFtrace To Trace VACUUM FULL Entries

Let’s start with a very simple BPFtrace program that prints a line once the vacuum_rel function is invoked in the PostgreSQL binary. bpftrace is called with the eBPF program that should be loaded into the Linux kernel. The eBPF programs that are passed to BPFtrace have the following syntax:

<probe1> {
        <Actions>
}

[...]

<probeN> {
        <Actions>
}

‌The syntax to define a UProbe on a binary or library is: uprobe:library_name:function_name[+offset]. For instance, to define a UProbe on the function invocation of vacuum_rel in the binary /home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres and print the line Vacuum started, you can use the following BPFtrace call:

$ sudo bpftrace -e '
uprobe:/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel {
    printf("Vacuum started\n");
}
'

Attaching 1 probe...
Vacuum started
Vacuum started
Vacuum started
Vacuum started
Vacuum started
Vacuum started
Vacuum started
Vacuum started
Vacuum started
[...]

As soon as the VACUUM FULL SQL statement in PostgreSQL is executed in another terminal session, the program starts to print the message on the screen. This is a good start, but we still have less information available than output by the existing tool funclatency-bpfcc. We're missing the latency of the function calls.

Tracing Vacuum Function Returns and Latency

To measure the execution time of the function invocations, we need two things:

  • Define a second Uprobe that is invoked when the function observed returns; this can be done by using a URetProbe.
  • Calculate the difference between the two UProbe events.

A URetProbe in BPFtrace can be defined using the same syntax uretprobe:binary:function as the one used for the UProbe. In addition, BPFtrace allows it to create variables like associative arrays.

We use such an array to capture the start time of a function invocation @start[tid] = nsecs;. The array's key is the ID of the current thread: tid. So, multiple threads (and processes like in our case with PostgreSQL) can be traced simultaneously without overriding the last function invitation start time.

In the URetProbe we take the current time and subtract the time of the function invocation (nsecs - @start[tid])) to get the time the function call needs. We also use a function predicate (/@start[tid]/) to let BPFtrace know that we only want to execute the function body of the URetProbe as soon as this array value is defined.

Using this predicate, we prevent handling a function return without seeing the function enter before (e.g., we start theBPFtrace program in the middle of a running function call, and we get only the URetProbe invocation for this function call).

Note: It is not guaranteed that BPFtrace will deliver and process the eBPF events in order. Especially when a function call is short, and we have a lot of function invocations, the events could be processed out-of-order (e.g., we see two function enter events followed by two function return events). In this case, function latency observations with BPFtrace become imprecise. To avoid this, we use VACUUM FULL calls instead of vacuum calls. These calls are much more expensive since they rewrite the table. Therefore, they take longer and can be reliably observed by BPFtrace.

$ sudo bpftrace -e '
uprobe:/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel
{
        printf("Performing vacuum\n");
        @start[tid] = nsecs;
}

uretprobe:/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel
/@start[tid]/
{
        printf("Vacuum call took %d ns\n", nsecs - @start[tid]);
        delete(@start[tid]);
}
'

After running this BPFtrace call and executing VACUUM FULL in a second session, we see the following output:

Attaching 2 probes...
Performing vacuum
Vacuum call took 37486735 ns
Performing vacuum
Vacuum call took 16491130 ns
Performing vacuum
Vacuum call took 32443568 ns
Performing vacuum
Vacuum call took 17959933 ns
[...]

‌For each call of the vacuum_rel function in PostgreSQL, we measure the time the vacuum operation needs. However, it would also be convenient to capture the Oid or the name of the relation vacuumed by the current vacuum operation. This requires the handling of the function parameters of the observed function.

Handle Function Parameters

The function vacuum_rel has the following signature in PostgreSQL 14. The first parameter is the Oid (an unsigned int) of the processed relation. The second parameter is a RangeVar struct, which could contain the name of the relation. The third parameter is a VacuumParams struct, which contains additional parameters for the vacuum operation, and the last parameter is a BufferAccessStrategy, which defines the access strategy of the used buffer.

static bool vacuum_rel(Oid relid,
        RangeVar *relation,
        VacuumParams *params,
        BufferAccessStrategy bstrategy 
)

BPFtrace allows it to access the function parameter using the keywords arg0, arg1, …, argN. To include the Oid in the output of our logging, we need only to print the first parameter of the function.

$ sudo bpftrace -e '

uprobe:/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel
{
        printf("Performing vacuum of Oid %d\n", arg0);
        @start[tid] = nsecs;
}

uretprobe:/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel
/@start[tid]/
{
        printf("Vacuum call took %d ns\n", nsecs - @start[tid]);
        delete(@start[tid]);
}
'

When the VACUUM FULL operation is executed again in a second terminal, the output looks as follows:

Attaching 2 probes...
[...]
Performing vacuum of Oid 1153888
Vacuum call took 37486734 ns
Performing vacuum of Oid 1153891
Vacuum call took 49535256 ns
Performing vacuum of Oid 2619
Vacuum call took 39575635 ns
Performing vacuum of Oid 2840
Vacuum call took 40683526 ns
Performing vacuum of Oid 1247
Vacuum call took 14683600 ns
Performing vacuum of Oid 4171
Vacuum call took 20587503 ns

‌To determine which Oid belongs to which relation, the following SQL statement can be executed:

blog=# SELECT oid, relname FROM pg_class WHERE oid IN (1153888, 1153891);
   oid   |  relname   
---------+------------
 1153888 | testtable1
 1153891 | testtable2
(2 rows)

The result shows that Oids 1153888 and 1153891 belong to the tables testtable1 and testtable2, which we have created in one of the first sections of this article. These values belong to our test environment. In your environment, different OIDs might be shown.

Handle Function Struct Parameters

So far, we have processed simple parameters with bpftrace (like Oids, which are unsigned integers). However, many parameters in PostgreSQL are C data structs. Furthermore, these structs can be handled in BPFtrace programs as well.

The second parameter of the vacuum_rel function is a RangeVar struct. This struct is defined in PostgreSQL 14 as follows:

typedef struct RangeVar
{
	NodeTag	type;
	char *catalogname;
	char *schemaname;
	char *relname;
	[...]
}

To process the struct, the following BPFtrace program can be used. Please note, that the internal NodeTag data type of PostgreSQL is replaced by a simple int.

The NodeTag data type is an enum. Enums are backed by the integer data type in C. To handle this enum correctly, we could (1) also copy the enum definition into the eBPF program, or (2) we could replace it with a data type of the same length.

To keep the BPFtrace program simple, the second option is used here. The next three struct members are char pointer, which contains the catalogname, the schema, and the name of the relation. The schemaname and the relname are the fields we are interested in. The struct contains more members, but these members are ignored to keep the example clear.

$ sudo bpftrace -e '
struct RangeVar
{
	int type;
	char *catalogname;
	char *schemaname;
	char *relname;
};

uprobe:/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel
{
        printf("[PID %d] Performing vacuum of Oid %d (%s.%s)\n", pid, arg0, str(((struct RangeVar*) arg1)->schemaname), str(((struct RangeVar*) arg1)->relname));
        @start[tid] = nsecs;
}

uretprobe:/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel
/@start[tid]/
{
        printf("[PID %d] Vacuum call took %d ns\n", pid, nsecs - @start[tid]);
        delete(@start[tid]);
}
'

After the struct is defined, the members of the struct can be accessed as in a regular C program. For example: ((struct RangeVar*) arg1)->schemaname. In addition, we also print the process ID (PID) of the program that has triggered the UProbe. This allows it to identify the process that has performed the vacuum operation.

When running the following SQL statements in a second terminal:

VACUUM FULL public.testtable1;
VACUUM FULL public.testtable2;

The BPFtrace program shows the following output:

Attaching 2 probes...
[PID 616516] Performing vacuum of Oid 1153888 (public.testtable1)
[PID 616516] Vacuum call took 23683600 ns
[PID 616516] Performing vacuum of Oid 1153891 (public.testtable2)
[PID 616516] Vacuum call took 24240837 ns

The table names are extracted from the RangeVar data structure and shown in the output. However, this data structure is not always populated by PostgreSQL. The data structure might be empty when running VACUUM FULL without specifying a table name. Therefore, we use two single invocations with explicit table names to force PostgreSQL to populate this data structure.

Optimizing the BPFtrace Program Using Maps

The BPFtrace programs we have developed so far use one or more printf statements directly. A printf call is slow and reduces the throughput the BPFtrace program can monitor.

This can be optimized by storing the data in map that is printed when BPFtrace is stopped. This will postpone the printf calls until observation is done. To do this, we introduce three new maps @start, @oid, and @vacuum. The first two maps are populated in the UProbe event of the vacuum_rel function. The map @start contains the time when the probe is triggered, and the map @oid contains the Oid of the parameter function.

When the function returns, and the URetProbe is triggered, the @vacuum map is populated. The key is the Oid, and the value is the needed time to perform the vacuum operation. Also, the keys of the first two maps are removed.

When BPFtrace exits (i.e., by pressing CRTL+C), all populated maps are printed automatically. By using these three maps (@start, @oid, @vaccum), we have separated the actual monitoring from the output, the expensive printf function is called after the monitoring is done.

In addition, in the following program, we use the two functions BEGIN and END that are called by BPFtrace when the observation begins and ends.

$ sudo sudo bpftrace -e '

uprobe:/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel
{
        @start[tid] = nsecs;
        @oid[tid] = arg0;
}

uretprobe:/home/jan/postgresql-sandbox/bin/REL_14_2_DEBUG/bin/postgres:vacuum_rel
/@start[tid]/
{

        @vacuum[@oid[tid]] = nsecs - @start[tid];
        delete(@start[tid]);
        delete(@oid[tid]);

}

BEGIN
{
        printf("VACUUM calles are traced, press CTRL+C to stop tracing\n");
}

END 
{
        printf("\n\nNeeded time in ns to perform VACUUM FULL per Oid\n");
}
'

After BPFtrace is started, the first message is printed. After the program is stopped, the second message is printed. Furthermore, the content of the @vacuum map is printed. For each Oid, the needed time for the vacuum operations is shown.

VACUUM calls are traced, press CTRL+C to stop tracing
^C

Needed time in ns to perform VACUUM FULL per Oid

@vacuum[1153888]: 7526823
@vacuum[1153891]: 8462672
@vacuum[2613]: 10764797
@vacuum[2995]: 11429589
@vacuum[6102]: 11436539
@vacuum[12801]: 14373934
@vacuum[6106]: 14396012
@vacuum[3118]: 14507167
@vacuum[3596]: 14695385
@vacuum[12811]: 14871237
@vacuum[3429]: 15106778
@vacuum[3350]: 15158742
@vacuum[2611]: 15432053
@vacuum[3764]: 15534169
@vacuum[2601]: 16055863
@vacuum[3602]: 16128624
@vacuum[2605]: 16405419
@vacuum[2616]: 16914195
@vacuum[3576]: 17003920
[...]

Conclusion

This article provides a brief overview of BPFtrace. To trace the function latency of PostgreSQL vacuum calls, we used the tool funclatency-bpfcc.

Additionally, we utilized BPFtrace to create a tool that allows for more in-depth observation of the calls. Our BPFtrace script also takes into account the parameters of the PostgreSQL vacuum_rel function, enabling us to monitor the vacuum time per relation.

And speaking of vacuum, check out this blog post to learn more about transaction ID wraparound exhaustion and how to avoid it in PostgreSQL databases.

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