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Re: [PATCH] Remove atomic operations from malloc.c

From: Leonhard Holz <leonhard dot holz at web dot de>
To: libc-alpha at sourceware dot org
Date: Wed, 11 Feb 2015 16:20:14 +0100
Subject: Re: [PATCH] Remove atomic operations from malloc.c
Authentication-results: sourceware.org; auth=none
References: <54DB130F dot 9070300 at web dot de> <1423652468 dot 9778 dot 250 dot camel at triegel dot csb> <54DB5011 dot 9020405 at linux dot vnet dot ibm dot com> <54DB594F dot 9070406 at web dot de> <54DB5D6D dot 5060305 at linux dot vnet dot ibm dot com>

Am 11.02.2015 um 14:47 schrieb Adhemerval Zanella:

On 11-02-2015 11:29, Leonhard Holz wrote:

I did get into the changes itself, but at least for powerpc (POWER8/16c/128T)
I am not seeing improvements with the patch.  In fact it seems to increase
contention:

             time per iteration
nths       master     patch
1           51.422    75.046
8          53.077    78.507
16         57.430    89.385
32         71.206   108.359
64        114.370   172.115
128       251.731   330.924


Thank you for testing! Maybe the costs of a mutex_lock are higher on PowerPC than on i686? Anyway it looks like I have to take a different approach...

PowerPC uses now the default implementation at sysdeps/nptl/lowlevellock.h which
basically translates to acquire CAS followed by a futex operation in contention
case.  So I think the gain is for powerpc (specially with high SMT), busy-wait
using like a spinlock yields better performance than possible issuing a futex
operations.


Indeed, using a spinlock for the fastbin path would be my next try.

References:
- [PATCH] Remove atomic operations from malloc.c
  - From: Leonhard Holz
- Re: [PATCH] Remove atomic operations from malloc.c
  - From: Torvald Riegel
- Re: [PATCH] Remove atomic operations from malloc.c
  - From: Adhemerval Zanella
- Re: [PATCH] Remove atomic operations from malloc.c
  - From: Leonhard Holz
- Re: [PATCH] Remove atomic operations from malloc.c
  - From: Adhemerval Zanella

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