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

[Zhong Yang]

On Fri, Nov 17, 2017 at 02:06:20PM +0000, Daniel P. Berrange wrote:
> 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.
  Hello Daniel,

  In fact, i firstly adopted mallopt() solution to optimize the heap memory,
  but i found this function is NOT useful, which are difference with MAN's
  description, so i had to swith to use malloc_trim().
  
  Regards,

  Yang

> Also, how does malloc_trim interact with tcmalloc.so that people often
> use in preference to glibc's built in malloc ?
  Thanks, you reminded me to consider tcmalloc or jemalloc.
 
  Whether below code is more suitable? thanks!
  #if defined(CONFIG_LINUX) && defined(__GLIBC__)
         malloc_trim(0);
  #endif

  Regards,

  Yang

> 
> Regards,
> Daniel
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