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Re: [Qemu-devel] [PATCH] rcu: reduce half heap memory size by malloc_tri
From: |
Zhong Yang |
Subject: |
Re: [Qemu-devel] [PATCH] rcu: reduce half heap memory size by malloc_trim() |
Date: |
Mon, 20 Nov 2017 16:54:42 +0800 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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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Re: [Qemu-devel] [PATCH] rcu: reduce half heap memory size by malloc_trim(), Fam Zheng, 2017/11/20