Re: [PATCH -v2 1/2] mm, swap: Use kvzalloc to allocate some swap data structure

[Huang\, Ying]

John Hubbard <jhubbard@xxxxxxxxxx> writes:

> On 03/23/2017 07:41 PM, Huang, Ying wrote:
>> David Rientjes <rientjes@xxxxxxxxxx> writes:
>>
>>> On Mon, 20 Mar 2017, Huang, Ying wrote:
>>>
>>>> From: Huang Ying <ying.huang@xxxxxxxxx>
>>>>
>>>> Now vzalloc() is used in swap code to allocate various data
>>>> structures, such as swap cache, swap slots cache, cluster info, etc.
>>>> Because the size may be too large on some system, so that normal
>>>> kzalloc() may fail.  But using kzalloc() has some advantages, for
>>>> example, less memory fragmentation, less TLB pressure, etc.  So change
>>>> the data structure allocation in swap code to use kvzalloc() which
>>>> will try kzalloc() firstly, and fallback to vzalloc() if kzalloc()
>>>> failed.
>>>>
>>>
>>> As questioned in -v1 of this patch, what is the benefit of directly
>>> compacting and reclaiming memory for high-order pages by first preferring
>>> kmalloc() if this does not require contiguous memory?
>>
>> The memory allocation here is only for swap on time, not for swap out/in
>> time.  The performance of swap on is not considered critical.  But if
>> the kmalloc() is used instead of the vmalloc(), the swap out/in
>> performance could be improved (marginally).  More importantly, the
>> interference for the other activity on the system could be reduced, For
>> example, less memory fragmentation, less TLB usage of swap subsystem,
>> etc.
>
> Hi Ying,
>
> I'm a little surprised to see vmalloc calls replaced with
> kmalloc-then-vmalloc calls, because that actually makes fragmentation
> worse (contrary to the above claim). That's because you will consume
> contiguous memory (even though you don't need it to be contiguous),
> whereas before, you would have been able to get by with page-at-a-time
> for vmalloc.
>
> So, things like THP will find fewer contiguous chunks, as a result of patches such as this.

Hi, John,

I don't think so.  The pages allocated by vmalloc() cannot be moved
during de-fragment.  For example, if 512 dis-continuous physical pages
are allocated via vmalloc(), at worst, one page will be allocate from
one distinct 2MB continous physical pages.  This makes 512 * 2MB = 1GB
memory cannot be used for THP allocation.  Because these pages cannot be
defragmented until vfree().

Best Regards,
Huang, Ying

> --
> thanks,
> john h
>
>>
>> Best Regards,
>> Huang, Ying
>>
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