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bug#43389: 28.0.50; Emacs memory leaks using hard disk all time

From: Eli Zaretskii
Subject: bug#43389: 28.0.50; Emacs memory leaks using hard disk all time
Date: Wed, 25 Nov 2020 20:03:35 +0200

> Cc: bugs@gnu.support, fweimer@redhat.com, 43389@debbugs.gnu.org,
>  dj@redhat.com, michael_heerdegen@web.de
> From: Carlos O'Donell <carlos@redhat.com>
> Date: Wed, 25 Nov 2020 12:45:04 -0500
> On 11/24/20 11:07 AM, Eli Zaretskii wrote:
> > Look at the large chunks in the tail of this.  Together, they do
> > account for ~2GB.
> > 
> > Carlos, are these chunks in use (i.e. allocated and not freed), or are
> > they the free chunks that are available for allocation, but not
> > released to the OS?  If the former, then it sounds like this session
> > does have around 2GB of allocated heap data, so either there's some
> > allocated memory we don't account for, or there is indeed a memory
> > leak in Emacs.  If these are the free chunks, then the way glibc
> > manages free'd memory is indeed an issue.
> These chunks are all free and mapped for use by the algorithm to satisfy
> a request by the application.

So we have more than 1.5GB free memory available for allocation, is
that right?

But then how to reconcile this with what you say next:

> <system type="current" size="4243079168"/>
> => Currently at 4.2GiB in arena 0 (kernel assigned heap).
> => The application is using that sbrk'd memory.
> <system type="max" size="4243079168"/>
> <aspace type="total" size="4243079168"/>
> <aspace type="mprotect" size="4243079168"/>
> => This indicates *real* API usage of 4.2GiB.

Here you seem to say that these 4.2GB are _used_ by the application?
While I thought the large chunks I asked about, which total more than
1.5GB, are a significant part of those 4.2GB?

To make sure there are no misunderstandings, I'm talking about this
part of the log:

  <heap nr="0">
    <size from="10753" to="12273" total="11387550" count="990"/>
    <size from="12289" to="16369" total="32661229" count="2317"/>
    <size from="16385" to="20465" total="36652437" count="2037"/>
    <size from="20481" to="24561" total="21272131" count="947"/>
    <size from="24577" to="28657" total="25462302" count="958"/>
    <size from="28673" to="32753" total="28087234" count="914"/>
    <size from="32769" to="36849" total="39080113" count="1121"/>
    <size from="36865" to="40945" total="30141527" count="775"/>
    <size from="40961" to="65521" total="166092799" count="3119"/>
    <size from="65537" to="98289" total="218425380" count="2692"/>
    <size from="98321" to="131057" total="178383171" count="1555"/>
    <size from="131089" to="163825" total="167800886" count="1142"/>
    <size from="163841" to="262065" total="367649915" count="1819"/>
    <size from="262161" to="522673" total="185347984" count="560"/>
    <size from="525729" to="30878897" total="113322865" count="97"/>
    <unsorted from="33" to="33" total="33" count="1"/>

If I sum up the "total=" parts of these large numbers, I get 1.6GB.
Is this free memory, given back to glibc for future allocations from
this arena, and if so, are those 1.6GB part of the 4.2GB total?

> This shows the application is USING memory on the main system heap.
> It might not be "leaked" memory since the application might be using it.
> You want visibility into what is USING that memory.
> With glibc-malloc-trace-utils you can try to do that with:
> LD_PRELOAD=libmtrace.so \
> MTRACE_CTL_FILE=/home/user/app.mtr \
> ./app
> This will use libgcc's unwinder to get a copy of the malloc caller
> address and then we'll have to decode that based on a /proc/self/maps.
> Next steps:
> - Get a glibc-malloc-trace-utils trace of the application ratcheting.
> - Get a copy of /proc/$PID/maps for the application (shorter version of 
> smaps).
> Then we might be able to correlate where all the kernel heap data went?

Thanks for the instructions.  Would people please try that and report
the results?

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