Subject: Re: bug#39413: 26.2; Emacs gets hung

From: "ISHIKAWA,chiaki" <[email-masked]>
Date: Wed, 11 Aug 2021 16:22:21 +0900

Hi,

Oops sorry, I forgot to attach the GC information dump when 2-min GC 
guru meditation time occurred.
Here it is. A copy from *Messages* buffer.
I am afraid that I had to back track to older messages and I believe the 
ones I copied here are related to the timing of the long GC.
But I could not exactly figure out which one.

Chiaki

<<< Type SPC or RET to bury the buffer list >>>
Mark set
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"Wed Aug 11 14:35:01 2021"
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"
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ESC C-M-p is undefined
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Saved text from "
"Wed Aug 11 14:25:39 2021"
"[(377509160"
(New file)
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Saving file /tmp/t.memory.txt...
Wrote /tmp/t.memory.txt
Making completion list...
Quit
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"Wed Aug 11 14:43:38 2021"
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"
C-M-g is undefined
Quit
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"Wed Aug 11 14:58:00 2021"
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On 2021/08/11 15:07, ISHIKAWA,chiaki wrote:
> (Sorry I am working from home and I have used my office e-mail address 
> and personal e-mail address interchangeably.)
>
> Hi,
>
> Good news. I could capture at least something that shows what happens 
> when the Emacs seems to spend strangely large amount of time in GC and 
> for all practical purposes it gets hung for a couple of minutes.
>
> With the post-gc-hook, usually "Garbage collecting ..." is overwritten 
> with the dump of array data that holds GC-related inforamtion. But if 
> GC is spending long time, I see "Garbage collecting" and hung emacs 
> window.
>
> Just immediately after I wrote my previous e-mail, I noticed that the 
> Mozilla repository corruption problem was finally solved. Official fix 
> was announced. So I applied it and luckily I could compile and build 
> mozilla thunderbird mail client for debugging again.
>
> And yes, the strange emacs hung or rather "GC is spending too much CPU 
> without visible change" happens again a few hours later, a few times.
>
> I am attaching two screen dump.
> - One is the screen capture of Linux desktop.
>    In it, you can see xosview window.
>    (I marked it with red pen.)
>    You can see memory is not used  up. Only about 11 GB of memory is 
> used out of 16 GB.
>    No swapping should occur.
>    Emacs is not responding at all since it is running wild in GC routine.
>   Thus its window cannot be repainted. (Blue pen mark.)
>    I had to move xosview window, etc. for better view and when the 
> windows got repainted,
>    Emacs window was not repainted. Other windows and programs such as 
> terminals are responding without any issue at all.
>
>    I could not measure the exact time Emacs was not responding. I 
> think it was like More thant1 min 30 seconds more likely over 2 minutes.
>
>    xosview has a CPU usage window which shows one CPU, CPU4  was 
> occupied hundred percent.
>    That was the CPU emacs uses.
>    The full horizontal span of data window covers about 30 seconds. So 
> you can see that GC (I think it is GC) runs more than 30 seconds by 
> then. Actually, after more than 30 seconds, I drifted off to capture 
> the screen, mark the captured image, etc. This more than 1 minute long 
> GC  is really puzzling.
>
> - The other attachment  is Windows task manager memory information.
>    Sorry the legends are in Japanese since I used Japanese Windows 
> locale.
>    You can see I still have like 4.5 G of free memory out of 32 GB 
> real memory.
>    Thus the host windows 10 is not feeling any problem and I can run 
> browser, e-mail client, etc. without any issue at all while Emacs is 
> running GC earnestly.
>
> I am attaching the GC dump data collected by my post-gc-hook function.
> But it does not reveal much.
>
> I probably should include the start of GC time and end of GC time in 
> the array of information I collect for each GC run. Then we will know 
> for sure how much time GC ran.
> But there does NOT seem to be a pre-gc-hook as opposed to post-gc-hook.
> Hmm... I wonder how I record the start time of GC call.
>
> Any suggestion on the last topic of how to record GC time at fine 
> granularity will be appreciated.
>
> Chiaki
>
> On 2021/08/11 10:08, ISHIKAWA,chiaki wrote:
>> On 2021/08/11 1:09, Lars Ingebrigtsen wrote:
>>> chiaki-ishikawa-thunderbird-account <[email-masked]> writes:
>>>
>>>> Stay tuned. I am really interested in WHERE/WHY this long pause
>>>> occurs.: maybe Windows 10 memory allocation/ Virtual Box
>>>> emulation/Linux kernel memory allocation, and AMD Ryzen CPU's cache
>>>> replacement policy all add up.
>>> This was a year ago -- are you still seeing these hangs, and if so, 
>>> have
>>> you confirmed that they don't happen because the machine (client or
>>> host) is swapping?
>>
>> Hi,
>>
>> Thank you for your follow-up.
>>
>> I still think there is a problem with memory allocator in the later 
>> Emacs versions. I can't figure out exactly when this happens. I have 
>> been using Emacs in this setup for quite sometime and yet I had NOT 
>> experienced this strange long pause before.
>>
>> Now, it is true that I do use much memory while running mozilla 
>> thunderbird compilation
>> WITHIN shell window which I open M-x shell.
>> But again, I used to do this for the last 8-9 years.
>>
>> One thing I notice is that running compilation script of mozilla 
>> thunderbird
>> outputs a pattern of longish lines and then a few short lines very 
>> frequently.
>> Yes, they amount to a lot of memory, but I suspect that
>> they may cause a somewhat inefficient handling by string allocator.
>>
>> I used the following post-GC-hook to record the behavior of emacs GC.
>>
>> After the code snippet, I attach the output from such a call.
>>
>> ;;;
>>
>> ;;; GC behavior monitor
>> ;;;
>> ;;; Record some key information for GC in the GC post-hook so that I
>> ;;; can learn something about the behavior and performance of GC with
>> ;;; concrete data.
>>
>> (setq my-gc-statistics (make-vector 30 nil))
>>
>> ;;; The element is
>> ;;; (append (memory-use-counts) (list gc-elapsed gcs-done))
>> ;;; Each time the following function is called, the
>> ;;; elements in the array is shifted toward the end.
>> ;;; Use (message "%S" my-gc-statistics) to force the
>> ;;; recording of my-gc-statistics value in *Messages* buffer for 
>> later analysis.
>>
>>
>> (defun update-my-gc-statistics ()
>>   (let ((i 28))
>>     (progn
>>      ;;; very unlike Lisp
>>      (while (<= 0 i)
>>        (progn (aset my-gc-statistics (+ 1 i) (aref my-gc-statistics i))
>>              (setq i (- i 1) )))
>>      (aset my-gc-statistics 0
>>            (append (memory-use-counts) (list gc-elapsed gcs-done)))
>>      ;;; print the latest one last so that I can see the glimpse in 
>> the narrow
>>      ;;; output window.
>>      (message "%S\n%S" (current-time-string) (pp (reverse 
>> my-gc-statistics))))))
>>
>> (setq post-gc-hook 'update-my-gc-statistics)
>>
>> ---
>> Normal behavior. Not much pause.
>>
>> [(19051645 7445 43993805 36764 42521186 286669 1176042 13.581105925 320)
>>  (19117025 7446 44152747 36764 42547677 287685 1177518 13.624215366 321)
>>  (19174133 7469 44332450 36765 42585061 288281 1178955 13.667294932 322)
>>  (19238776 7470 44504363 36765 42620253 289199 1180384 13.714802148 323)
>>  (19303795 7484 44666173 36767 42649518 289786 1181923 13.756323136 324)
>>  (19365595 7485 44837589 36767 42676862 290659 1183423 13.807570787 325)
>>  (19425828 7486 45007513 36767 42712688 291410 1184879 13.847687952 326)
>>  (19482572 7488 45186533 36769 42753576 291969 1186503 13.905988177 327)
>>  (19539282 7489 45365763 36769 42782042 292534 1188002 13.947620008 328)
>>  (19610699 7526 45530194 36769 42832733 293078 1189542 13.98705147 329)
>>  (19677333 7536 45705015 36769 42861041 293694 1190941 14.041105235 330)
>>  (19736527 7537 45875148 36769 42896534 294340 1192316 14.082710146 331)
>>  (19799864 7538 46047037 36769 42922173 295201 1193738 14.126554401 332)
>>  (19865128 7542 46209994 36769 42949055 295876 1195133 14.167501197 333)
>>  (19924904 7564 46377456 36769 43008388 296531 1196605 14.207478738 334)
>>  (19986717 7674 46546221 36770 43035046 296928 1197955 14.249106813 335)
>>  (20046726 7675 46716423 36770 43060757 297558 1199338 14.302346074 336)
>>  (20107508 7678 46888051 36772 43097195 298290 1200743 14.342108851 337)
>>  (20172065 7680 47061015 36774 43126993 298770 1202285 14.383165327 338)
>>  (20235780 7681 47221594 36774 43153041 299635 1203754 14.423942963 339)
>>  (20294623 7682 47405064 36774 43202163 300434 1205260 14.473857509 340)
>>  (20353428 7692 47574958 36778 43250345 300773 1207056 14.528225012 341)
>>  (20427274 7778 47679351 36778 43337331 303690 1212437 14.570319341 342)
>>  (20501106 7941 47776024 36778 43443248 307419 1218097 14.610816267 343)
>>  (20566988 7996 47933001 36778 43490727 309071 1221003 14.675236804 344)
>>  (20637587 8078 48085242 36778 43532391 309493 1222405 14.716701768 345)
>>  (20697606 8127 48240788 36778 43601564 310907 1225370 14.759790056 346)
>>  (20762697 8186 48384470 36778 43646718 312881 1228282 14.799301233 347)
>>  (20816911 8199 48566522 36778 43672496 313533 1229651 14.863568335 348)
>>  (20885113 8221 48719120 36778 43710277 314237 1231072 14.904231536 
>> 349)]
>>
>>
>> Well, due to a build issue (corrupt repository caused by MERCURIAL bug.
>> https://bugzilla.mozilla.org/show_bug.cgi?id=1720302
>> I have not been able to do much build and debug work on mozilla TB 
>> for the last few weeks.
>>
>> But when I do run the compilation, etc. I still see the strange 
>> pause/hang from time to time.
>> Only Emacs is affected. Other programs within virtualbox responds,.
>> Repainting of emacs does not happen.
>> Also, Windows 10 host is running smoothly. So I suspect Emacs is 
>> running CPU wildly when this happens. Yes, I have seen CPU usage go 
>> up using xosview.
>>
>> There may be some unexpected inefficiency ( a coding error that may 
>> cause an extra loop over a gigabyte of memoyr?)
>>
>> When I try to analyze the issue, it is always (well, it seems to me) 
>> related to string allocation.
>>
>> I have no idea where to look for the issue. As I said, the particular 
>> pattern of allocation by shell-mode and a lot of output from 
>> compilation script (with short and long lines mixed)  may cause some 
>> strange inefficiency in memory access behavior.
>>
>> Chiaki
>>
>>
>>
>