"Allocations of larger pages are not reliable in Linux. If larger pages have to be allocated then one faces various choices of allowing graceful fallback or using vmalloc with a performance penalty due to the use of a page table," began Christoph Lameter, describing the third version of his virtual compound page support patchset. He continued, "a virtual compound allocation means that there will be first of all an attempt to satisfy the request with physically contiguous memory. If that is not possible then a virtually contiguous memory will be created." Christopher proposed two advantages:
"1. Current uses of vmalloc can be converted to allocate virtual compounds instead. In most cases physically contiguous memory can be used which avoids the vmalloc performance penalty. 2. Uses of higher order allocations (stacks, buffers etc) can be converted to use virtual compounds instead. Physically contiguous memory will still be used for those higher order allocs in general but the system can degrade to the use of vmalloc should memory become heavily fragmented."
From: Christoph Lameter
Subject: [00/14] Virtual Compound Page Support V3
Date: Mar 20, 11:17 pm 2008
Allocations of larger pages are not reliable in Linux. If larger
pages have to be allocated then one faces various choices of allowing
graceful fallback or using vmalloc with a performance penalty due
to the use of a page table. Virtual Compound pages are
a simple solution out of this dilemma.
A virtual compound allocation means that there will be first of all
an attempt to satisfy the request with physically contiguous memory.
If that is not possible then a virtually contiguous memory will be
created.
This has two advantages:
1. Current uses of vmalloc can be converted to allocate virtual
compounds instead. In most cases physically contiguous
memory can be used which avoids the vmalloc performance
penalty. See f.e. the e1000 driver patch.
2. Uses of higher order allocations (stacks, buffers etc) can be
converted to use virtual compounds instead. Physically contiguous
memory will still be used for those higher order allocs in general
but the system can degrade to the use of vmalloc should memory
become heavily fragmented.
There is a compile time option to switch on fallback for
testing purposes. Virtually mapped mmemory may behave differently
and the CONFIG_FALLBACK_ALWAYS option will ensure that the code is
tested to deal with virtual memory.
V2->V3:
- Put the code into mm/vmalloc.c and leave the page allocator alone.
- Add a series of examples where virtual compound pages can be used.
- Diffed on top of the page flags and the vmalloc info patches
already in mm.
- Simplify things by omitting some of the more complex code
that used to be in there.
V1->V2
- Remove some cleanup patches and the SLUB patches from this set.
- Transparent vcompound support through page_address() and
virt_to_head_page().
- Additional use cases.
- Factor the code better for an easier read
- Add configurable stack size.
- Follow up on various suggestions made for V1
RFC->V1
- Complete support for all compound functions for virtual compound pages
(including the compound_nth_page() necessary for LBS mmap support)
- Fix various bugs
- Fix i386 build
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[patch 1] [patch 2] [patch 3] [patch 4] [patch 5] [patch 6] [patch 7] [patch 8] [patch 9] [patch 10] [patch 11] [patch 12] [patch 13] [patch 14]
What remains for higher order allocs?
If I understand correctly, the only thing that remains are higher-order allocs for physical memory that needs to operate in terms of physical addresses, such as DMAs, correct?
--
Program Intellivision and play Space Patrol!
actually
Yesterday I read on lwn.net/Kernel/ there is a patch to handle DMA differently, specific for DMA.