linux-kernel - Re: [External] Re: Basics : Memory Configuration

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Message-ID: <SG2PR02MB30989B644196598CEDDD5337E82E0@SG2PR02MB3098.apcprd02.prod.outlook.com>
Date:   Wed, 10 Apr 2019 06:06:36 +0000
From:   Pankaj Suryawanshi <pankaj.suryawanshi@...fochips.com>
To:     Christopher Lameter <cl@...ux.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [External] Re: Basics : Memory Configuration

________________________________________
From: Christopher Lameter <cl@...ux.com>
Sent: 09 April 2019 21:31
To: Pankaj Suryawanshi
Cc: linux-kernel@...r.kernel.org; linux-mm@...ck.org
Subject: [External] Re: Basics : Memory Configuration

On Tue, 9 Apr 2019, Pankaj Suryawanshi wrote:

> I am confuse about memory configuration and I have below questions

Hmmm... Yes some of the terminology that you use is a bit confusing.

> 1. if 32-bit os maximum virtual address is 4GB, When i have 4 gb of ram
> for 32-bit os, What about the virtual memory size ? is it required
> virtual memory(disk space) or we can directly use physical memory ?

The virtual memory size is the maximum virtual size of a single process.
Multiple processes can run and each can use different amounts of physical
memory. So both are actually independent.

The size of the virtual memory space per process is configurable on x86 32
bit (2G, 3G, 4G). Thus the possible virtual process size may vary
depending on the hardware architecture and the configuration of the
kernel.

If i have configures VMSPLIT = 2G/2G what does it mean ?
Virtual memory uses disk space ? let say for 32-bit os i have 4GB ram than what is the use case of virtual memory ?

If i have 32-bit and 2gb/3gb ram than virtual memory is useful  because its less than 4GB ?

> 2. In 32-bit os 12 bits are offset because page size=4k i.e 2^12 and
> 2^20 for page addresses
>    What about 64-bit os, What is offset size ? What is page size ? How it calculated.

12 bits are passed through? Thats what you mean?

The remainder of the bits  are used to lookup the physical frame
number(PFN) in the page tables.

64 bit is the same. However, the number of bits used for lookups in the
page tables are much higher.

for 32-bit os page size is 4k, what is the page size for 64-bit os ? page size and offset is related to each other ?

if i increase the page size from 4k to 8k, does it change the offset size that it 2^12 to 2^13 ?

Why only 48 bits are used in 64-bit os ?

> 3. What is PAE? If enabled how to decide size of PAE, what is maximum
> and minimum size of extended memory.

PAE increases the physical memory size that can be addressed through a
page table lookup. The number of bits that can be specified in the PFN is
increased and thus more than 4GB of physical memory can be used by the
operating system. However, the virtual memory size stays the same and an
individual process still cannot use more memory.

Let say i have ,enabled PAE for 32-bit os with 6GB ram.Virtual size is same 4GB, 32-bit os cant address more thatn 4gb, Than what is the use of 6GB with PAE enabled.

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