From: Minchan Kim <minchan@kernel.org>
To: Nitin Gupta <ngupta@vflare.org>
Cc: Pekka Enberg <penberg@kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Seth Jennings <sjenning@linux.vnet.ibm.com>,
Dan Magenheimer <dan.magenheimer@oracle.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
cl@linux-foundation.org
Subject: Re: [PATCH 4/4] zsmalloc: zsmalloc: align cache line size
Date: Tue, 08 May 2012 10:34:47 +0900 [thread overview]
Message-ID: <4FA87837.3050208@kernel.org> (raw)
In-Reply-To: <4FA7C2BC.2090400@vflare.org>
On 05/07/2012 09:40 PM, Nitin Gupta wrote:
> On 5/7/12 3:41 AM, Pekka Enberg wrote:
>> On Fri, 4 May 2012, Minchan Kim wrote:
>>>>> It's a overkill to align pool size with PAGE_SIZE to avoid
>>>>> false-sharing. This patch aligns it with just cache line size.
>>>>>
>>>>> Signed-off-by: Minchan Kim<minchan@kernel.org>
>>>>> ---
>>>>> drivers/staging/zsmalloc/zsmalloc-main.c | 6 +++---
>>>>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c
>>>>> b/drivers/staging/zsmalloc/zsmalloc-main.c
>>>>> index 51074fa..3991b03 100644
>>>>> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
>>>>> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
>>>>> @@ -489,14 +489,14 @@ fail:
>>>>>
>>>>> struct zs_pool *zs_create_pool(const char *name, gfp_t flags)
>>>>> {
>>>>> - int i, error, ovhd_size;
>>>>> + int i, error;
>>>>> struct zs_pool *pool;
>>>>>
>>>>> if (!name)
>>>>> return NULL;
>>>>>
>>>>> - ovhd_size = roundup(sizeof(*pool), PAGE_SIZE);
>>>>> - pool = kzalloc(ovhd_size, GFP_KERNEL);
>>>>> + pool = kzalloc(ALIGN(sizeof(*pool), cache_line_size()),
>>>>> + GFP_KERNEL);
>>>>
>>>> a basic question:
>>>> Is rounding off allocation size to cache_line_size enough to ensure
>>>> that the object is cache-line-aligned? Isn't it possible that even
>>>> though the object size is multiple of cache-line, it may still not be
>>>> properly aligned and end up sharing cache line with some other
>>>> read-mostly object?
>>>
>>> AFAIK, SLAB allocates object aligned cache-size so I think that
>>> problem cannot happen.
>>> But needs double check.
>>> Cced Pekka.
>>
>> The kmalloc(size) function only gives you the following guarantees:
>>
>> (1) The allocated object is _at least_ 'size' bytes.
>>
>> (2) The returned pointer is aligned to ARCH_KMALLOC_MINALIGN.
>>
>> Anything beyond that is implementation detail and probably will break if
>> you switch between SLAB/SLUB/SLOB.
>>
>> Pekka
Pekka, Thanks.
>
> So, we can probably leave it as is (PAGE_SIZE aligned) or use
> kmem_cache_create(...,SLAB_HWCACHE_ALIGN,...) for allocating 'struct
> zs_pool's.
3) remove aligning code totally because there isn't any report about degradation by false-sharing.
4)
origin = pool = kzalloc(sizeof(*pool) + cache_line_size, GFP_KERNEL);
pool = round_up(pool, cache_line_size);
Which preference?
I choose 3.
>
> zcache can potentially create a lot of pools, so the latter will save
> some memory.
Dumb question.
Why should we create pool per user?
What's the problem if there is only one pool in system?
>
> Thanks,
> Nitin
>
> --
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--
Kind regards,
Minchan Kim
next prev parent reply other threads:[~2012-05-08 1:34 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-05-03 6:40 [PATCH 1/4] zsmalloc: rename zspage_order with zspage_pages Minchan Kim
2012-05-03 6:40 ` [PATCH 2/4] zsmalloc: add/fix function comment Minchan Kim
2012-05-03 13:19 ` Nitin Gupta
2012-05-03 6:40 ` [PATCH 3/4] zsmalloc use zs_handle instead of void * Minchan Kim
2012-05-03 13:32 ` Nitin Gupta
2012-05-03 15:23 ` Seth Jennings
2012-05-04 2:24 ` Minchan Kim
2012-05-07 15:01 ` Seth Jennings
2012-05-09 20:19 ` Greg Kroah-Hartman
2012-05-10 2:03 ` Minchan Kim
2012-05-10 14:02 ` Konrad Rzeszutek Wilk
2012-05-10 14:12 ` Greg Kroah-Hartman
2012-05-10 14:47 ` Nitin Gupta
2012-05-10 15:00 ` Minchan Kim
2012-05-10 15:11 ` Seth Jennings
2012-05-10 15:19 ` Minchan Kim
2012-05-10 15:19 ` Greg Kroah-Hartman
2012-05-10 16:29 ` Nitin Gupta
2012-05-10 16:44 ` Greg Kroah-Hartman
2012-05-10 17:24 ` Nitin Gupta
2012-05-10 17:33 ` Konrad Rzeszutek Wilk
2012-05-10 23:24 ` Minchan Kim
2012-05-10 23:50 ` Dan Magenheimer
2012-05-11 0:14 ` Minchan Kim
2012-05-11 16:31 ` Dan Magenheimer
2012-05-11 19:29 ` Konrad Rzeszutek Wilk
2012-05-11 21:49 ` Seth Jennings
2012-05-14 2:26 ` Minchan Kim
2012-05-11 19:28 ` Konrad Rzeszutek Wilk
2012-05-14 2:18 ` Minchan Kim
2012-05-15 1:57 ` Nitin Gupta
2012-05-15 2:21 ` Minchan Kim
2012-05-15 15:04 ` Konrad Rzeszutek Wilk
2012-05-16 1:36 ` Minchan Kim
2012-05-16 11:06 ` Konrad Rzeszutek Wilk
2012-05-03 6:40 ` [PATCH 4/4] zsmalloc: zsmalloc: align cache line size Minchan Kim
2012-05-03 13:58 ` Nitin Gupta
2012-05-04 2:27 ` Minchan Kim
2012-05-07 7:41 ` Pekka Enberg
2012-05-07 12:40 ` Nitin Gupta
2012-05-08 1:34 ` Minchan Kim [this message]
2012-05-08 14:00 ` Dan Magenheimer
2012-05-09 0:58 ` Minchan Kim
2012-05-09 3:08 ` Dan Magenheimer
2012-05-09 4:07 ` Minchan Kim
2012-05-11 0:03 ` Dan Magenheimer
2012-05-11 0:25 ` Minchan Kim
2012-05-11 19:06 ` Konrad Rzeszutek Wilk
2012-05-14 1:55 ` Minchan Kim
2012-05-15 15:18 ` Konrad Rzeszutek Wilk
2012-05-16 1:44 ` Minchan Kim
2012-05-03 13:18 ` [PATCH 1/4] zsmalloc: rename zspage_order with zspage_pages Nitin Gupta
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