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From: "Huang\, Ying" <ying.huang@intel.com>
To: David Rientjes <rientjes@google.com>
Cc: "Huang\, Ying" <ying.huang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Shaohua Li <shli@kernel.org>, Rik van Riel <riel@redhat.com>,
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        Gerald Schaefer <gerald.schaefer@de.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>, "Ingo Molnar" <mingo@kernel.org>,
        Vegard Nossum <vegard.nossum@oracle.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH -v2 1/2] mm, swap: Use kvzalloc to allocate some swap data structure
References: <20170320084732.3375-1-ying.huang@intel.com>
        <alpine.DEB.2.10.1703201430550.24991@chino.kir.corp.google.com>
Date: Fri, 24 Mar 2017 10:41:45 +0800
In-Reply-To: <alpine.DEB.2.10.1703201430550.24991@chino.kir.corp.google.com>
        (David Rientjes's message of "Mon, 20 Mar 2017 14:32:27 -0700")
Message-ID: <8737e3z992.fsf@yhuang-dev.intel.com>
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David Rientjes <rientjes@google.com> writes:

> On Mon, 20 Mar 2017, Huang, Ying wrote:
>
>> From: Huang Ying <ying.huang@intel.com>
>> 
>> 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.

Best Regards,
Huang, Ying