Subject: Re: bug#46881: 28.0.50; pdumper dumping causes way too many syscalls

From: Eli Zaretskii <[email-masked]>
Date: Wed, 03 Mar 2021 07:51:05 +0200

> From: Pip Cet <[email-masked]>
> Date: Tue, 2 Mar 2021 20:45:04 +0000
> 
> On Tue, Mar 2, 2021 at 8:35 PM Pip Cet <[email-masked]> wrote:
> > I've looked into the problem, and it seems easy to solve and worth it
> > in terms of debuggability and performance.
> 
> Very rough benchmarks, but this seems to be clearly worth it:
> 
> Performance:
> With patch:
> real    0m3.861s
> user    0m3.776s
> sys    0m0.085s
> 
> Without patch:
> real    0m7.001s
> user    0m4.476s
> sys    0m2.511s
> 
> Number of syscalls:
> With patch: 415442
> Without patch: 2028307
> 
> > Patch will be attached once this has a bug number.
> 
> And here's the patch. Testing would be very appreciated.
> 
> I'm unsure about the precise usage of dump_off vs ptrdiff_t here; I
> don't think it matters, but suggestions, nitpicks, and comments, on
> this or any other aspect, would be very appreciated.

> From 92ee138852b34ede2f43dd7f93f310fc746bb3bf Mon Sep 17 00:00:00 2001
> From: Pip Cet <[email-masked]>
> Date: Tue, 2 Mar 2021 20:38:23 +0000
> Subject: [PATCH] Prepare pdumper dump file in memory, write it in one go
>  (Bug#46881)
> 
> * src/pdumper.c (struct dump_context): Add buf, buf_size, max_offset fields.
> (grow_buffer): New function.
> (dump_write): Use memcpy, not an actual emacs_write.
> (dump_seek): Keep track of maximum seen offset.
> (Fdump_emacs_portable): Write out the file contents when done.
> ---
>  src/pdumper.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/src/pdumper.c b/src/pdumper.c
> index 337742fda4ade..62ddad8ee5e34 100644
> --- a/src/pdumper.c
> +++ b/src/pdumper.c
> @@ -473,6 +473,10 @@ dump_fingerprint (char const *label,
>  {
>    /* Header we'll write to the dump file when done.  */
>    struct dump_header header;
> +  /* Data that will be written to the dump file.  */
> +  void *buf;
> +  ptrdiff_t buf_size;
> +  ptrdiff_t max_offset;
>  
>    Lisp_Object old_purify_flag;
>    Lisp_Object old_post_gc_hook;
> @@ -581,6 +585,13 @@ dump_fingerprint (char const *label,
>  
>  /* Dump file creation */
>  
> +static void dump_grow_buffer (struct dump_context *ctx)
> +{
> +  ctx->buf = xrealloc (ctx->buf, ctx->buf_size = (ctx->buf_size ?
> +						  (ctx->buf_size * 2)
> +						  : 1024 * 1024));
> +}
> +
>  static dump_off dump_object (struct dump_context *ctx, Lisp_Object object);
>  static dump_off dump_object_for_offset (struct dump_context *ctx,
>  					Lisp_Object object);
> @@ -747,8 +758,9 @@ dump_write (struct dump_context *ctx, const void *buf, dump_off nbyte)
>    eassert (nbyte == 0 || buf != NULL);
>    eassert (ctx->obj_offset == 0);
>    eassert (ctx->flags.dump_object_contents);
> -  if (emacs_write (ctx->fd, buf, nbyte) < nbyte)
> -    report_file_error ("Could not write to dump file", ctx->dump_filename);
> +  while (ctx->offset + nbyte > ctx->buf_size)
> +    dump_grow_buffer (ctx);
> +  memcpy ((char *)ctx->buf + ctx->offset, buf, nbyte);
>    ctx->offset += nbyte;
>  }
>  
> @@ -828,6 +840,8 @@ dump_tailq_pop (struct dump_tailq *tailq)
>  static void
>  dump_seek (struct dump_context *ctx, dump_off offset)
>  {
> +  if (ctx->max_offset < ctx->offset)
> +    ctx->max_offset = ctx->offset;
>    eassert (ctx->obj_offset == 0);
>    if (lseek (ctx->fd, offset, SEEK_SET) < 0)
>      report_file_error ("Setting file position",
> @@ -4159,6 +4173,8 @@ DEFUN ("dump-emacs-portable",
>    ctx->header.magic[0] = dump_magic[0];
>    dump_seek (ctx, 0);
>    dump_write (ctx, &ctx->header, sizeof (ctx->header));
> +  if (emacs_write (ctx->fd, ctx->buf, ctx->max_offset) < ctx->max_offset)
> +    report_file_error ("Could not write to dump file", ctx->dump_filename);
>  
>    dump_off
>      header_bytes = header_end - header_start,
> -- 
> 2.30.1

Thanks.

Daniel, Paul: any comments?  In particular, is it safe to allocate
large amounts of memory off the heap while dumping?  A couple of
places in pdumper.c says some parts of code should call malloc.