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

From: Daniel Colascione <[email-masked]>
Date: Thu, 4 Mar 2021 17:26:32 -0500

On 3/3/21 12:51 AM, Eli Zaretskii wrote:

>> 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.

It looks fine, but wouldn't dumping to a FILE* (with internal buffering) 
do the same basic thing in a simpler way? There aren't any particular 
constraints on the environment _during_ the dump: we even make new lisp 
objects. It's when loading the dump, early in initialization, that you 
have to be careful.