Re: [Qemu-devel] [PATCH] rcu: reduce half heap memory size by malloc_trim()

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

On Fri, Nov 17, 2017 at 01:54:09PM +0000, Stefan Hajnoczi wrote:
> On Fri, Nov 17, 2017 at 02:23:34PM +0800, Yang Zhong wrote:
> > diff --git a/util/rcu.c b/util/rcu.c
> > index ca5a63e..8d491a6 100644
> > --- a/util/rcu.c
> > +++ b/util/rcu.c
> > @@ -26,6 +26,7 @@
> >   * IBM's contributions to this file may be relicensed under LGPLv2 or later.
> >   */
> >  
> > +#include <malloc.h>
> 
> This header file is not mentioned in the C99 standard or POSIX.  It is
> probably not available on all host OSes that QEMU supports.  Please use
> #ifdef CONFIG_LINUX.
> 
> >  #include "qemu/osdep.h"
> >  #include "qemu-common.h"
> >  #include "qemu/rcu.h"
> > @@ -272,6 +273,9 @@ static void *call_rcu_thread(void *opaque)
> >              node->func(node);
> >          }
> >          qemu_mutex_unlock_iothread();
> > +#ifdef CONFIG_LINUX
> > +        malloc_trim(0);
> > +#endif
> 
> It is important that the rcu thread isn't overzealous in minimizing heap
> size if that means ordinary malloc(3) calls will experience latency
> spikes.  Please leave a few MB free so that malloc(3) doesn't take the
> slow path.

If you pass '0' the docs say that the minimum amount is left in the
heap, per M_TOP_PAD, which is 128kb.

Strangely the mallopt(3) man page suggests, that free() should automatically
trim the heap when its size exceeds M_TOP_TRIM, which is again 128kb by
default.  So I'm puzzelled by malloc_trim() would be needed unless there
are scenarios in which free() won't trim, that aren't mentioned in the
manpage.

Also, how does malloc_trim interact with tcmalloc.so that people often
use in preference to glibc's built in malloc ?

Regards,
Daniel
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