From: Simon Marchi <simon.marchi@polymtl.ca>
To: Aditya Vidyadhar Kamath <akamath996@gmail.com>,
ulrich.weigand@de.ibm.com, tom@tromey.com
Cc: gdb-patches@sourceware.org, Aditya.Kamath1@ibm.com,
sangamesh.swamy@in.ibm.com
Subject: Re: [PATCH 2/3][RFC] Batch getthrds() calls in get_signaled_thread
Date: Wed, 16 Sep 2026 14:34:38 -0400 [thread overview]
Message-ID: <cb25db63-7045-4f1f-90a5-4aae0344cce4@polymtl.ca> (raw)
In-Reply-To: <20260916114603.17488-4-akamath996@gmail.com>
On 2026-09-16 07:46, Aditya Vidyadhar Kamath wrote:
> From: Aditya Vidyadhar Kamath <aditya.kamath1@ibm.com>
>
> The original code called getthrds() with a count of 1, fetching one
> kernel thread descriptor per syscall. With N threads in the process
> that costs N syscalls per stop just to find the signaled thread.
>
> Change to fetch GETTHRDS_BATCH (64) descriptors per call and scan
> the batch in a loop. This reduces the syscall count from O(N) to
> O(N/64).
>
> Also done the benchmark here who results are pasted below. To analyse
> where time is being spent. For now, this patch request for comment/draft and will
> remove the same and this paragraph from the commit message after
> feedback.
>
> ==========
> Benchmark C code
> // cat ../bench_next.c
> /* Spawns NUM_THREADS worker threads that spin, then steps through
> 10 assignments in do_steps() so GDB can time each next.
>
> Also tests two scenarios from the bug report:
> 1. "next" over a call that spawns a thread (spawn_thread_here()).
> 2. A background thread exits while "next" is in progress
> (exit_thread_here()). */
> #include <stdio.h>
> #include <stdlib.h>
> #include <pthread.h>
> #include <unistd.h>
>
> #ifndef NUM_THREADS
> #define NUM_THREADS 3
> #endif
>
> static volatile int keep_running = 1;
> static volatile int extra_thread_started = 0;
>
> static void *
> worker (void *arg)
> {
> while (keep_running)
> sched_yield ();
> return NULL;
> }
>
> /* Thread that exits on its own after signalling it has started. */
> static void *
> short_lived (void *arg)
> {
> extra_thread_started = 1;
> return NULL;
> }
>
> /* GDB will "next" over this call -- a new thread is spawned inside. */
> static void
> spawn_thread_here (void)
> {
> pthread_t t;
> pthread_create (&t, NULL, short_lived, NULL);
> pthread_join (t, NULL);
> }
>
> /* GDB will "next" over this call -- the short-lived background thread
> is in the process of exiting while we step. */
> static void
> exit_thread_here (void)
> {
> /* Just a visible marker so GDB can stop here. */
> volatile int x = 42;
> (void)x;
> }
>
> void
> do_steps (void)
> {
> volatile int a = 1;
> volatile int b = 2;
> volatile int c = a + b;
> volatile int d = c * 2;
> volatile int e = d - a;
> volatile int f = e + c;
> volatile int g = f / 2;
> volatile int h = g + 1;
> volatile int i2 = h * h;
> volatile int j = i2 - b;
> (void)j;
> }
>
> int
> main (void)
> {
> pthread_t threads[NUM_THREADS];
> pthread_t bg;
> int i;
>
> /* Start background spinning threads. */
> for (i = 0; i < NUM_THREADS; i++)
> pthread_create (&threads[i], NULL, worker, NULL);
>
> /* Next over a call that spawns a thread */
> spawn_thread_here ();
>
> /* Next while a background thread is exiting
> Launch a thread that exits quickly; "next" through exit_thread_here()
> while that exit is in flight. */
> pthread_create (&bg, NULL, short_lived, NULL);
> exit_thread_here ();
> pthread_join (bg, NULL);
>
> do_steps ();
>
> keep_running = 0;
> for (i = 0; i < NUM_THREADS; i++)
> pthread_join (threads[i], NULL);
>
> return 0;
> }
I would expect the benchmark program to have a ton of
threads, to show what a worst case scenario of get_signaled_thread
looks like. This program appears to spawn 3 threads max, that means
before the patch you'd get 3-4 getthrds calls, and after the patch you'd
get 1. I suppose it would be more interesting to see what happens when
you have e.g. 1000 threads, in which case the number of getthrds calls
goes from 1000 to 16.
Also, since there is an early return in get_signaled_thread, if you want
to show the worst case, you'd need to set things up such that the
signaled thread is in the last batch, ideally the last thread (assuming
the order in which the kernel returns threads is predictable).
> ============
> Benchmark GDB code:
>
> // cat ../bench_next.gdb
> set pagination off
> set confirm off
> maintenance set per-command time on
>
> # Scenario 1: next over a call that spawns a thread
> break spawn_thread_here
> run
> next
>
> # Scenario 2: next while a background thread is exiting
> break exit_thread_here
> continue
> next
>
> break do_steps
> continue
>
> next
> next
> next
> next
> next
> next
> next
> next
> next
> next
>
> quit
>
> ===============
> # cd /current_gdb && time /current_gdb/binutils-gdb/gdb/gdb -batch -x bench_next.gdb ./bench_next_bin 2>&1 | grep -E "^(real|user|sys)|volatile int
> |pd_update total|sync_thread|pthdb_session"
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "pd_update total": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "pd_update total": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.146, user 0.081, sys 0.085, user+sys 0.166, 14.5 % CPU
> Time for "pd_update total": wall 1.146, user 0.081, sys 0.085, user+sys 0.166, 14.5 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.025, user 0.092, sys 0.092, user+sys 0.184, 18.0 % CPU
> Time for "pd_update total": wall 1.025, user 0.092, sys 0.092, user+sys 0.184, 18.0 % CPU
> 25 volatile int a = 1;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.962, user 0.098, sys 0.095, user+sys 0.193, 20.1 % CPU
> Time for "pd_update total": wall 0.962, user 0.098, sys 0.095, user+sys 0.193, 20.1 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.965, user 0.098, sys 0.094, user+sys 0.192, 19.9 % CPU
> Time for "pd_update total": wall 0.965, user 0.098, sys 0.094, user+sys 0.192, 19.9 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.954, user 0.098, sys 0.094, user+sys 0.192, 20.1 % CPU
> Time for "pd_update total": wall 0.954, user 0.098, sys 0.094, user+sys 0.192, 20.1 % CPU
> 26 volatile int b = 2;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.949, user 0.098, sys 0.094, user+sys 0.192, 20.2 % CPU
> Time for "pd_update total": wall 0.949, user 0.098, sys 0.094, user+sys 0.192, 20.2 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.957, user 0.098, sys 0.094, user+sys 0.192, 20.1 % CPU
> Time for "pd_update total": wall 0.957, user 0.098, sys 0.094, user+sys 0.192, 20.1 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.033, user 0.101, sys 0.098, user+sys 0.199, 19.3 % CPU
> Time for "pd_update total": wall 1.033, user 0.101, sys 0.098, user+sys 0.199, 19.3 % CPU
> 27 volatile int c = a + b;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.031, user 0.101, sys 0.098, user+sys 0.199, 19.3 % CPU
> Time for "pd_update total": wall 1.031, user 0.101, sys 0.098, user+sys 0.199, 19.3 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.028, user 0.100, sys 0.098, user+sys 0.198, 19.3 % CPU
> Time for "pd_update total": wall 1.028, user 0.100, sys 0.098, user+sys 0.198, 19.3 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.996, user 0.100, sys 0.098, user+sys 0.198, 19.9 % CPU
> Time for "pd_update total": wall 0.996, user 0.100, sys 0.098, user+sys 0.198, 19.9 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.000, user 0.098, sys 0.098, user+sys 0.196, 19.6 % CPU
> Time for "pd_update total": wall 1.001, user 0.098, sys 0.098, user+sys 0.196, 19.6 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.998, user 0.098, sys 0.098, user+sys 0.196, 19.6 % CPU
> Time for "pd_update total": wall 0.998, user 0.098, sys 0.098, user+sys 0.196, 19.6 % CPU
> 28 volatile int d = c * 2;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.999, user 0.098, sys 0.098, user+sys 0.196, 19.6 % CPU
> Time for "pd_update total": wall 0.999, user 0.098, sys 0.098, user+sys 0.196, 19.6 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.001, user 0.099, sys 0.098, user+sys 0.197, 19.7 % CPU
> Time for "pd_update total": wall 1.001, user 0.099, sys 0.098, user+sys 0.197, 19.7 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.969, user 0.100, sys 0.095, user+sys 0.195, 20.1 % CPU
> Time for "pd_update total": wall 0.969, user 0.100, sys 0.095, user+sys 0.195, 20.1 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.980, user 0.099, sys 0.096, user+sys 0.195, 19.9 % CPU
> Time for "pd_update total": wall 0.980, user 0.099, sys 0.096, user+sys 0.195, 19.9 % CPU
> 29 volatile int e = d - a;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.984, user 0.100, sys 0.096, user+sys 0.196, 19.9 % CPU
> Time for "pd_update total": wall 0.984, user 0.100, sys 0.096, user+sys 0.196, 19.9 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.094, user 0.098, sys 0.094, user+sys 0.192, 17.6 % CPU
> Time for "pd_update total": wall 1.094, user 0.099, sys 0.094, user+sys 0.193, 17.6 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.019, user 0.106, sys 0.097, user+sys 0.203, 19.9 % CPU
> Time for "pd_update total": wall 1.019, user 0.106, sys 0.097, user+sys 0.203, 19.9 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.002, user 0.106, sys 0.096, user+sys 0.202, 20.2 % CPU
> Time for "pd_update total": wall 1.002, user 0.106, sys 0.096, user+sys 0.202, 20.2 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.003, user 0.105, sys 0.096, user+sys 0.201, 20.0 % CPU
> Time for "pd_update total": wall 1.003, user 0.105, sys 0.096, user+sys 0.201, 20.0 % CPU
> 30 volatile int f = e + c;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.011, user 0.106, sys 0.097, user+sys 0.203, 20.1 % CPU
> Time for "pd_update total": wall 1.011, user 0.106, sys 0.097, user+sys 0.203, 20.1 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.010, user 0.107, sys 0.096, user+sys 0.203, 20.1 % CPU
> Time for "pd_update total": wall 1.010, user 0.107, sys 0.096, user+sys 0.203, 20.1 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.026, user 0.110, sys 0.096, user+sys 0.206, 20.1 % CPU
> Time for "pd_update total": wall 1.026, user 0.110, sys 0.096, user+sys 0.206, 20.1 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.200, user 0.090, sys 0.085, user+sys 0.175, 14.6 % CPU
> Time for "pd_update total": wall 1.201, user 0.090, sys 0.085, user+sys 0.175, 14.6 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.212, user 0.090, sys 0.091, user+sys 0.181, 14.9 % CPU
> Time for "pd_update total": wall 1.212, user 0.090, sys 0.091, user+sys 0.181, 14.9 % CPU
> 31 volatile int g = f / 2;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 0.993, user 0.103, sys 0.096, user+sys 0.199, 20.0 % CPU
> Time for "pd_update total": wall 0.993, user 0.103, sys 0.096, user+sys 0.199, 20.0 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.258, user 0.079, sys 0.082, user+sys 0.161, 12.8 % CPU
> Time for "pd_update total": wall 1.258, user 0.079, sys 0.082, user+sys 0.161, 12.8 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.255, user 0.080, sys 0.081, user+sys 0.161, 12.8 % CPU
> Time for "pd_update total": wall 1.255, user 0.080, sys 0.081, user+sys 0.161, 12.8 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.259, user 0.080, sys 0.081, user+sys 0.161, 12.8 % CPU
> Time for "pd_update total": wall 1.259, user 0.080, sys 0.081, user+sys 0.161, 12.8 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.264, user 0.080, sys 0.082, user+sys 0.162, 12.8 % CPU
> Time for "pd_update total": wall 1.265, user 0.080, sys 0.082, user+sys 0.162, 12.8 % CPU
> 32 volatile int h = g + 1;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.261, user 0.079, sys 0.083, user+sys 0.162, 12.8 % CPU
> Time for "pd_update total": wall 1.261, user 0.079, sys 0.083, user+sys 0.162, 12.8 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.232, user 0.076, sys 0.081, user+sys 0.157, 12.7 % CPU
> Time for "pd_update total": wall 1.232, user 0.076, sys 0.081, user+sys 0.157, 12.7 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.250, user 0.078, sys 0.083, user+sys 0.161, 12.9 % CPU
> Time for "pd_update total": wall 1.250, user 0.078, sys 0.083, user+sys 0.161, 12.9 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.258, user 0.080, sys 0.084, user+sys 0.164, 13.0 % CPU
> Time for "pd_update total": wall 1.258, user 0.080, sys 0.084, user+sys 0.164, 13.0 % CPU
> 33 volatile int i2 = h * h;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.276, user 0.082, sys 0.082, user+sys 0.164, 12.9 % CPU
> Time for "pd_update total": wall 1.276, user 0.082, sys 0.082, user+sys 0.164, 12.9 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.271, user 0.083, sys 0.081, user+sys 0.164, 12.9 % CPU
> Time for "pd_update total": wall 1.271, user 0.083, sys 0.081, user+sys 0.164, 12.9 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.280, user 0.079, sys 0.086, user+sys 0.165, 12.9 % CPU
> Time for "pd_update total": wall 1.280, user 0.079, sys 0.086, user+sys 0.165, 12.9 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.211, user 0.075, sys 0.081, user+sys 0.156, 12.9 % CPU
> Time for "pd_update total": wall 1.211, user 0.075, sys 0.081, user+sys 0.156, 12.9 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.264, user 0.077, sys 0.086, user+sys 0.163, 12.9 % CPU
> Time for "pd_update total": wall 1.264, user 0.077, sys 0.086, user+sys 0.163, 12.9 % CPU
> 34 volatile int j = i2 - b;
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.240, user 0.078, sys 0.081, user+sys 0.159, 12.8 % CPU
> Time for "pd_update total": wall 1.240, user 0.078, sys 0.081, user+sys 0.159, 12.8 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.256, user 0.078, sys 0.082, user+sys 0.160, 12.7 % CPU
> Time for "pd_update total": wall 1.257, user 0.078, sys 0.082, user+sys 0.160, 12.7 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.267, user 0.078, sys 0.082, user+sys 0.160, 12.6 % CPU
> Time for "pd_update total": wall 1.267, user 0.078, sys 0.082, user+sys 0.160, 12.6 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.254, user 0.079, sys 0.081, user+sys 0.160, 12.8 % CPU
> Time for "pd_update total": wall 1.255, user 0.079, sys 0.081, user+sys 0.160, 12.7 % CPU
> Time for "pthdb_session_update": wall 0.000, user 0.000, sys 0.000, user+sys 0.000, NaNQ % CPU
> Time for "sync_threadlists": wall 1.257, user 0.078, sys 0.082, user+sys 0.160, 12.7 % CPU
> Time for "pd_update total": wall 1.257, user 0.078, sys 0.082, user+sys 0.160, 12.7 % CPU
I don't see any "Time for" line for get_signaled_thread, which is the
function you are optimizing in this patch... am I missing something?
Simon
prev parent reply other threads:[~2026-09-16 18:35 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-16 11:46 Aditya Vidyadhar Kamath
2026-09-16 18:34 ` Simon Marchi [this message]
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