Re: [00/17] [RFC] Virtual Compound Page Support
On Wed, 19 Sep 2007 08:34:47 +0100
Anton Altaparmakov [EMAIL PROTECTED] wrote:
Hi Christoph,
On 19 Sep 2007, at 04:36, Christoph Lameter wrote:
Currently there is a strong tendency to avoid larger page
allocations in
the kernel because of past fragmentation issues and the current
defragmentation methods are still evolving. It is not clear to what
extend
they can provide reliable allocations for higher order pages (plus the
definition of reliable seems to be in the eye of the beholder).
Currently we use vmalloc allocations in many locations to provide a
safe
way to allocate larger arrays. That is due to the danger of higher
order
allocations failing. Virtual Compound pages allow the use of regular
page allocator allocations that will fall back only if there is an
actual
problem with acquiring a higher order page.
This patch set provides a way for a higher page allocation to fall
back.
Instead of a physically contiguous page a virtually contiguous page
is provided. The functionality of the vmalloc layer is used to provide
the necessary page tables and control structures to establish a
virtually
contiguous area.
I like this a lot. It will get rid of all the silly games we have to
play when needing both large allocations and efficient allocations
where possible. In NTFS I can then just allocated higher order pages
instead of having to mess about with the allocation size and
allocating a single page if the requested size is = PAGE_SIZE or
using vmalloc() if the size is bigger. And it will make it faster
because a lot of the time a higher order page allocation will succeed
with your patchset without resorting to vmalloc() so that will be a
lot faster.
So where I currently have fs/ntfs/malloc.h the below mess I could get
rid of it completely and just use the normal page allocator/
deallocator instead...
static inline void *__ntfs_malloc(unsigned long size, gfp_t gfp_mask)
{
if (likely(size = PAGE_SIZE)) {
BUG_ON(!size);
/* kmalloc() has per-CPU caches so is faster for
now. */
return kmalloc(PAGE_SIZE, gfp_mask ~__GFP_HIGHMEM);
/* return (void *)__get_free_page(gfp_mask); */
}
if (likely(size PAGE_SHIFT num_physpages))
return __vmalloc(size, gfp_mask, PAGE_KERNEL);
return NULL;
}
And other places in the kernel can make use of the same. I think XFS
does very similar things to NTFS in terms of larger allocations at
least and there are probably more places I don't know about off the
top of my head...
I am looking forward to your patchset going into mainline. (-:
Sure, it sounds *really* good. But...
1) Only power of two allocations are good candidates, or we waste RAM
2) On i386 machines, we have a small vmalloc window. (128 MB default value)
Many servers with 4GB memory (PAE) like to boot with vmalloc=32M option to
get 992MB of LOWMEM.
If we allow some slub caches to fallback to vmalloc land, we'll have problems
to tune this.
3) A fallback to vmalloc means an allocation of one vm_struct per compound page.
4) vmalloc() currently uses a linked list of vm_struct. Might need something
more scalable.
-
To unsubscribe from this list: send the line unsubscribe linux-fsdevel in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [00/17] [RFC] Virtual Compound Page Support
On Wed, 19 Sep 2007, Eric Dumazet wrote: 1) Only power of two allocations are good candidates, or we waste RAM Correct. 2) On i386 machines, we have a small vmalloc window. (128 MB default value) Many servers with 4GB memory (PAE) like to boot with vmalloc=32M option to get 992MB of LOWMEM. If we allow some slub caches to fallback to vmalloc land, we'll have problems to tune this. We would first do the vmalloc conversion to GFP_VFALLBACK which would reduce the vmalloc requirements of drivers and core significantly. The patchset should actually reduce the vmalloc space requirements significantly. They are only needed in situations where the page allocator cannot provide a contiguous mapping and that gets rarer the better Mel's antifrag code works. 4) vmalloc() currently uses a linked list of vm_struct. Might need something more scalable. If its rarely used then its not that big of a deal. The better the anti fragmentation measures the less vmalloc use. - To unsubscribe from this list: send the line unsubscribe linux-fsdevel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [00/17] [RFC] Virtual Compound Page Support
On Wed, 19 Sep 2007, Andi Kleen wrote: alloc_page(GFP_VFALLBACK, order, ) Is there a reason this needs to be a GFP flag versus a wrapper around alloc_page/free_page ? page_alloc.c is already too complicated and it's better to keep new features separated. The only drawback would be that free_pages would need a different call, but that doesn't seem like a big problem. I tried to make this a wrapper but there is a lot of logic in __alloc_pages() that would have to be replicated. Also there are specific places in __alloc_pages() were we can establish that we have enough memory but its the memory fragmentation that prevents us from satisfying the request for a larger page. I'm also a little dubious about your attempts to do vmalloc in interrupt context. Is that really needed? GFP_ATOMIC allocations of large areas seem to be extremly unreliable to me and not design. Even if it works sometimes free probably wouldn't work there due to the flushes, which is very nasty. It would be better to drop that. The flushes are only done on virtuall mapped architectures (xtensa) and are simple ASM code that can run in an interrupt context - To unsubscribe from this list: send the line unsubscribe linux-fsdevel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
[00/17] [RFC] Virtual Compound Page Support
Currently there is a strong tendency to avoid larger page allocations in the kernel because of past fragmentation issues and the current defragmentation methods are still evolving. It is not clear to what extend they can provide reliable allocations for higher order pages (plus the definition of reliable seems to be in the eye of the beholder). Currently we use vmalloc allocations in many locations to provide a safe way to allocate larger arrays. That is due to the danger of higher order allocations failing. Virtual Compound pages allow the use of regular page allocator allocations that will fall back only if there is an actual problem with acquiring a higher order page. This patch set provides a way for a higher page allocation to fall back. Instead of a physically contiguous page a virtually contiguous page is provided. The functionality of the vmalloc layer is used to provide the necessary page tables and control structures to establish a virtually contiguous area. Advantages: - If higher order allocations are failing then virtual compound pages consisting of a series of order-0 pages can stand in for those allocations. - Reliability as long as the vmalloc layer can provide virtual mappings. - Ability to reduce the use of vmalloc layer significantly by using physically contiguous memory instead of virtual contiguous memory. Most uses of vmalloc() can be converted to page allocator calls. - The use of physically contiguous memory instead of vmalloc may allow the use larger TLB entries thus reducing TLB pressure. Also reduces the need for page table walks. Disadvantages: - In order to use fall back the logic accessing the memory must be aware that the memory could be backed by a virtual mapping and take precautions. virt_to_page() and page_address() may not work and vmalloc_to_page() and vmalloc_address() (introduced through this patch set) may have to be called. - Virtual mappings are less efficient than physical mappings. Performance will drop once virtual fall back occurs. - Virtual mappings have more memory overhead. vm_area control structures page tables, page arrays etc need to be allocated and managed to provide virtual mappings. The patchset provides this functionality in stages. Stage 1 introduces the basic fall back mechanism necessary to replace vmalloc allocations with alloc_page(GFP_VFALLBACK, order, ) which signifies to the page allocator that a higher order is to be found but a virtual mapping may stand in if there is an issue with fragmentation. Stage 1 functionality does not allow allocation and freeing of virtual mappings from interrupt contexts. The stage 1 series ends with the conversion of a few key uses of vmalloc in the VM to alloc_pages() for the allocation of sparsemems memmap table and the wait table in each zone. Other uses of vmalloc could be converted in the same way. Stage 2 functionality enhances the fallback even more allowing allocation and frees in interrupt context. SLUB is then modified to use the virtual mappings for slab caches that are marked with SLAB_VFALLBACK. If a slab cache is marked this way then we drop all the restraints regarding page order and allocate good large memory areas that fit lots of objects so that we rarely have to use the slow paths. Two slab caches--the dentry cache and the buffer_heads--are then flagged that way. Others could be converted in the same way. The patch set also provides a debugging aid through setting CONFIG_VFALLBACK_ALWAYS If set then all GFP_VFALLBACK allocations fall back to the virtual mappings. This is useful for verification tests. The test of this patch set was done by enabling that options and compiling a kernel. Stage 3 functionality could be the adding of support for the large buffer size patchset. Not done yet and not sure if it would be useful to do. Much of this patchset may only be needed for special cases in which the existing defragmentation methods fail for some reason. It may be better to have the system operate without such a safety net and make sure that the page allocator can return large orders in a reliable way. The initial idea for this patchset came from Nick Piggin's fsblock and from his arguments about reliability and guarantees. Since his fsblock uses the virtual mappings I think it is legitimate to generalize the use of virtual mappings to support higher order allocations in this way. The application of these ideas to the large block size patchset etc are straightforward. If wanted I can base the next rev of the largebuffer patchset on this one and implement fallback. Contrary to Nick, I still doubt that any of this provides a guarantee. Have said that I have to deal with various failure scenarios in the VM daily and I'd certainly like to see it work in a more reliable manner. IMHO getting rid of the various workarounds to deal with the small 4k pages and avoiding additional layers that group these pages in subsystem specific
