Re: Removal of unexec support from glibc malloc

[Florian Weimer <fweimer@redhat.com>]

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On 01/19/2016 02:44 AM, Ken Raeburn wrote:
> 
>> On Jan 18, 2016, at 20:14, Dmitry Gutov <dgutov@yandex.ru> wrote:
>>
>> On 01/19/2016 01:45 AM, Ali Bahrami wrote:
>>
>>> Before you fight to to save unexec, I'd encourage you to measure the
>>> impact, and see if it still matters.
>>
>> As a rough measurement, 'src/emacs -Q' takes ~0.5s to launch on my machine. 'src/temacs -Q` takes ~5.5s.
>>
>> It seems not only machines have become faster, Emacs has grown quite a bit, too. :)
> 
> It’s also pretty aggressive about garbage collection after loading any code before the unexec.  Presumably that’s to minimize the storage needed for the shared on-disk image.
> 
> Try removing the after-load-functions hook in loadup.el that invokes GC, and the explicit GC calls.  Also maybe bump up gc-cons-threshold by an order of magnitude.  (Just until loading finishes, if you like.)

The attached patch brings down the temacs load time to under 0.5 seconds.

The major offender is find_string_data_in_pure in src/alloc.c, and
setting purify to nil gets rid of that.  Some additional savings come
from removing the after-load-function hook in loadup.el, as you
suggested, but they are comparatively minor.

For some reason, international/characters does not load anymore, so I
dropped it for quick testing.

Florian

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diff --git a/lisp/loadup.el b/lisp/loadup.el
index 5f29c01..8870eb7 100644
--- a/lisp/loadup.el
+++ b/lisp/loadup.el
@@ -47,6 +47,8 @@
 
 ;;; Code:
 
+(setq purify-flag nil)
+
 ;; Add subdirectories to the load-path for files that might get
 ;; autoloaded when bootstrapping.
 ;; This is because PATH_DUMPLOADSEARCH is just "../lisp".
@@ -99,10 +101,6 @@
 (load "emacs-lisp/backquote")
 (load "subr")
 
-;; Do it after subr, since both after-load-functions and add-hook are
-;; implemented in subr.el.
-(add-hook 'after-load-functions (lambda (f) (garbage-collect)))
-
 (load "version")
 
 (load "widget")
@@ -168,7 +166,6 @@
 ;; This file doesn't exist when building a development version of Emacs
 ;; from the repository.  It is generated just after temacs is built.
 (load "international/charprop.el" t)
-(load "international/characters")
 (load "composite")
 
 ;; Load language-specific files.
@@ -374,11 +371,6 @@
 ;; Avoid storing references to build directory in the binary.
 (setq custom-current-group-alist nil)
 
-;; We keep the load-history data in PURE space.
-;; Make sure that the spine of the list is not in pure space because it can
-;; be destructively mutated in lread.c:build_load_history.
-(setq load-history (mapcar 'purecopy load-history))
-
 (set-buffer-modified-p nil)
 
 (remove-hook 'after-load-functions (lambda (f) (garbage-collect)))
@@ -413,10 +405,6 @@
     (message "Pure-hashed: %d strings, %d vectors, %d conses, %d bytecodes, %d others"
              strings vectors conses bytecodes others)))
 
-;; Avoid error if user loads some more libraries now and make sure the
-;; hash-consing hash table is GC'd.
-(setq purify-flag nil)
-
 (if (null (garbage-collect))
     (setq pure-space-overflow t))