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wal_system.py
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wal_system.py
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#!/usr/bin/env python
#
# wal_system Summarize amount of WAL written from the system point of view.
# For that purpose we track pwrite and check stack trace to come
# from wal write operation. Relying on stack trace may cause some
# missing results, and if it's a significant issue, try
# wal_per_query.
#
# usage: wal_system $PG_BIN/postgres [-d] [-p PID]
from __future__ import print_function
import argparse
import signal
from time import sleep
from bcc import BPF
import utils
# load BPF program
bpf_text = """
#include <linux/ptrace.h>
#include <uapi/linux/bpf_perf_event.h>
#define HASH_SIZE 2^14
#define QUERY_LEN 100
#define STACK_STORAGE_SIZE 1024
struct key_t {
int cpu;
u32 pid;
u64 namespace;
int user_stack_id;
char name[TASK_COMM_LEN];
};
struct backend {
int pid;
char query[QUERY_LEN];
};
BPF_HASH(query_stacks, struct key_t, int);
BPF_HASH(queries, u32, struct backend, HASH_SIZE);
BPF_STACK_TRACE(stack_traces, STACK_STORAGE_SIZE);
int syscall__pwrite(struct bpf_perf_event_data *ctx) {
struct key_t key = {};
key.pid = bpf_get_current_pid_tgid();
key.user_stack_id = stack_traces.get_stackid(&ctx->regs, BPF_F_USER_STACK);
SAVE_NAMESPACE
CHECK_NAMESPACE
bpf_get_current_comm(&(key.name), sizeof(key.name));
int len = (int) PT_REGS_PARM3(&ctx->regs);
query_stacks.update(&key, &len);
int zero = 0, *val;
val = query_stacks.lookup_or_init(&key, &zero);
(*val) += len;
return 0;
}
void probe_exec_simple_query(struct pt_regs *ctx, const char *query_string)
{
u32 pid = bpf_get_current_pid_tgid();
struct backend data = {};
data.pid = pid;
bpf_probe_read(&data.query, QUERY_LEN, &(*query_string));
queries.update(&pid, &data);
}
void probe_exec_simple_query_finish(struct pt_regs *ctx)
{
u32 pid = bpf_get_current_pid_tgid();
queries.delete(&pid);
}
"""
# signal handler
def signal_ignore(sig, frame):
print()
def attach(bpf, args):
bpf.attach_uprobe(
name=args.path,
sym="exec_simple_query",
fn_name="probe_exec_simple_query",
pid=args.pid)
bpf.attach_uretprobe(
name=args.path,
sym="exec_simple_query",
fn_name="probe_exec_simple_query_finish",
pid=args.pid)
bpf.attach_kprobe(
event="sys_pwrite64",
fn_name="syscall__pwrite")
def print_stack(bpf, stack_id, tgid):
if stack_id < 0:
return
stack = list(bpf.get_table("stack_traces").walk(stack_id))
for addr in stack:
print(" ", end="")
print("%16x " % addr, end="")
print("%s" % (bpf.sym(addr, tgid)))
def stack_from_wal(bpf, stack_id, tgid):
if stack_id < 0:
return False
stack = list(bpf.get_table("stack_traces").walk(stack_id))
for addr in stack:
sym = bpf.sym(addr, tgid)
if sym in {b"XLogBackgroundFlush", b"XLogFlush"}:
return True
return False
def pre_process(text, args):
text = utils.replace_namespace(text, args)
return text
def output(bpf, fmt="plain"):
if fmt == "plain":
print()
for (k, v) in bpf.get_table('query_stacks').items():
if stack_from_wal(bpf, k.user_stack_id, k.pid):
print("[{}:{}], {}, len: {}".format(
k.pid, k.namespace, k.name, utils.size(v.value)))
print_stack(bpf, k.user_stack_id, k.pid)
def run(args):
print("Attaching...")
debug = 4 if args.debug else 0
bpf = BPF(text=pre_process(bpf_text, args), debug=debug)
attach(bpf, args)
exiting = False
print("Listening...")
while True:
try:
sleep(args.interval)
output(bpf)
bpf.get_table('query_stacks').clear()
if args.debug:
bpf.perf_buffer_poll()
except KeyboardInterrupt:
exiting = True
# as cleanup can take many seconds, trap Ctrl-C:
signal.signal(signal.SIGINT, signal_ignore)
if exiting:
print()
print("Detaching...")
print()
break
output(bpf)
def parse_args():
parser = argparse.ArgumentParser(
description="Amount of WAL written from system point of view",
formatter_class=argparse.RawDescriptionHelpFormatter)
parser.add_argument("path", type=str, help="path to PostgreSQL binary")
parser.add_argument(
"-p", "--pid", type=int, default=-1,
help="trace this PID only")
parser.add_argument(
"-c", "--container", type=str,
help="trace this container only")
parser.add_argument(
"-n", "--namespace", type=int,
help="trace this namespace only")
parser.add_argument(
"-i", "--interval", type=int, default=5,
help="after how many seconds output the result")
parser.add_argument(
"-d", "--debug", action='store_true', default=False,
help="debug mode")
return parser.parse_args()
if __name__ == "__main__":
run(parse_args())