Re: Regarding memory fragmentation using malloc....

From: Michal Nazarewicz
Date: Thu Apr 14 2011 - 06:48:04 EST

On Thu, 14 Apr 2011 08:44:50 +0200, Pintu Agarwal <pintu_agarwal@xxxxxxxxx> wrote:
As I can understand from your comments that, malloc from user space will not have much impact on memory fragmentation.

It has an impact, just like any kind of allocation, it just don't care about
fragmentation of physical memory. You can have only 0-order pages and
successfully allocate megabytes of memory with malloc().

Will the memory fragmentation be visible if I do kmalloc from
the kernel module????

It will be more visible in the sense that if you allocate 8 KiB, kernel will
have to find 8 KiB contiguous physical memory (ie. 1-order page).

No. When you call malloc() only virtual address space is allocated.
The actual allocation of physical space occurs when user space accesses
the memory (either reads or writes) and it happens page at a time.

Here, if I do memset then I am accessing the memory...right? That I am doing already in my sample program.

Yes. But note that even though it's a single memset() call, you are
accessing page at a time and kernel is allocating page at a time.

On some architectures (not ARM) you could access two pages with a single
instructions but I think that would result in two page faults anyway. I
might be wrong though, the details are not important though.

what really happens is that kernel allocates the 0-order
pages and when
it runs out of those, splits a 1-order page into two
0-order pages and
takes one of those.

Actually, if I understand buddy allocator, it allocates pages from top to bottom.

No. If you want to allocate a single 0-order page, buddy looks for a
a free 0-order page. If one is not found, it will look for 1-order page
and split it. This goes up till buddy reaches (MAX_ORDER-1)-page.

Is the memory fragmentation is always a cause of the kernel space program and not user space at all?

Well, no. If you allocate memory in user space, kernel will have to
allocate physical memory and *every* allocation may contribute to
fragmentation. The point is, that all allocations from user-space are
single-page allocations even if you malloc() MiBs of memory.

Can you provide me with some references for migitating memory fragmentation in linux?

I'm not sure what you mean by that.

Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michal "mina86" Nazarewicz (o o)
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