Re: [PATCH] misc: introduce strim-memory qapi to support free memory trimming

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Jul 25, 2024 at 01:35:21PM +0200, Markus Armbruster wrote:
> Guoyi Tu <tugy@chinatelecom.cn> writes:
> 
> > In the test environment, we conducted IO stress tests on all storage disks
> > within a virtual machine that had five storage devices mounted.During 
> > testing,
> > we found that the qemu process allocated a large amount of memory (~800MB)
> > to handle these IO operations.
> >
> > When the test ended, although qemu called free() to release the allocated
> > memory, the memory was not actually returned to the operating system, as
> > observed via the top command.
> >
> > Upon researching the glibc memory management mechanism, we found that when
> > small chunks of memory are allocated in user space and then released with
> > free(),  the glibc memory management mechanism does not necessarily return
> > this memory to the operating system. Instead, it retains the memory until
> > certain conditions are met for release.
> 
> Yes.

Looking at mallopt(3) man page, the M_TRIM_THRESHOLD is said to control
when glibc releases the top of the heap back to the OS. It is said to
default to 128 kb.

I'm curious how we get from that default, to 800 MB of unused memory ?
Is it related to the number of distinct malloc arenas that are in use ?

I'm curious what malloc_stats() would report before & after malloc_trim
when QEMU is in this situation with lots of wasted memory.

> 
> > For virtual machines that only have business operations during specific
> > periods,  they remain idle most of the time. However, the qemu process
> > still occupies a large amount of memory resources, leading to significant
> > memory resource waste.
> 
> Mitigation: the memory free()'s but not returned to the OS can be paged
> out.
> 
> > To address this issue, this patch introduces an API to actively reclaim
> > idle memory within the qemu process. This API effectively calls 
> > malloc_trim()
> > to notify glibc to trim free memory. With this api, the management tool
> > can monitor the virtual machine's state and call this API during idle times
> > to free up the memory occupied by the virtual machine, thereby allowing more
> > virtual machines to be provisioned.
> 
> How does this affect the test case you described above?
> 
> There's an existing use of malloc_trim() in util/rcu.c's
> call_rcu_thread().  It's from commit 5a22ab71623:
> 
>     rcu: reduce more than 7MB heap memory by malloc_trim()
>     
>     Since there are some issues in memory alloc/free machenism
>     in glibc for little chunk memory, if Qemu frequently
>     alloc/free little chunk memory, the glibc doesn't alloc
>     little chunk memory from free list of glibc and still
>     allocate from OS, which make the heap size bigger and bigger.
>     
>     This patch introduce malloc_trim(), which will free heap
>     memory when there is no rcu call during rcu thread loop.
>     malloc_trim() can be enabled/disabled by --enable-malloc-trim/
>     --disable-malloc-trim in the Qemu configure command. The
>     default malloc_trim() is enabled for libc.
>     
>     Below are test results from smaps file.
>     (1)without patch
>     55f0783e1000-55f07992a000 rw-p 00000000 00:00 0  [heap]
>     Size:              21796 kB
>     Rss:               14260 kB
>     Pss:               14260 kB
>     
>     (2)with patch
>     55cc5fadf000-55cc61008000 rw-p 00000000 00:00 0  [heap]
>     Size:              21668 kB
>     Rss:                6940 kB
>     Pss:                6940 kB
>     
>     Signed-off-by: Yang Zhong <yang.zhong@intel.com>
>     Message-Id: <1513775806-19779-1-git-send-email-yang.zhong@intel.com>
>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> 
> How would the malloc_trim() you propose interact with this one?

The above usage is automatic, while this proposal requires that
an external mgmt app monitor QEMU and tell it to free memory.
I'm wondering if the latter is really desirable, or whether QEMU
can call this itself when reasonable ?


With regards,
Daniel
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