Re: [RFC] Reimplementation of linux dynamic percpu memory allocator

From: Ravikiran G Thirumalai
Date: Mon Dec 20 2004 - 13:10:40 EST

On Fri, Dec 17, 2004 at 11:29:42PM +0100, Manfred Spraul wrote:
> >
> Probably the right approach. slab should use per-cpu for it's internal
> head arrays, but I've never converted the slab code due to
> chicken-and-egg problems and due to the additional pointer dereference.
> >+ * Allocator is slow -- expected to be called during module/subsytem
> >+ * init. alloc_percpu can block.
> >
> >
> How slow is slow?

Haven't measured it, but the allocator is not designed for speed.
Once the block to be alloced from is identified, the allocator builds
and sorts a map of objects in ascending order so that we allocate
from the smallest chunk. Goal is to enhance memory/cacheline utlization
and reduce fragmentation rather than speed. It is not expected that
the allocator will be used from the fastpath.

> I think the block subsystem uses alloc_percpu for some statistics
> counters, i.e. one alloc during creation of a new disk. The slab
> implementation was really slow and that caused problems with LVM (or
> something like that) stress tests.

Hmmm..I knew from some experiments earlier that access to per cpu versions
of memory was slow with the slab based implementation -- which this patch
addresses, but I didn't know allocs themselves were slow...
Creation of a disk should not be a fast path no?

> >+ /* Map pages for each cpu by splitting vm_struct for each cpu */
> >+ for (i = 0; i < NR_CPUS; i++) {
> >+ if (cpu_possible(i)) {
> >+ tmppage = &blkp->pages[i*cpu_pages];
> >+ tmp.addr = area->addr + i * PCPU_BLKSIZE;
> >+ /* map_vm_area assumes a guard page of size
> >+ tmp.size = PCPU_BLKSIZE + PAGE_SIZE;
> >+ if (map_vm_area(&tmp, PAGE_KERNEL, &tmppage))
> >+ goto fail_map;
> >
> >
> That means no large pte entries for the per-cpu allocations, right?
> I think that's a bad idea for non-numa systems. What about a fallback to
> simple getfreepages() for non-numa systems?

Can we have large pte entries with PAGE_SIZEd pages?

> >+ * This allocator is slow as we assume allocs to come
> >+ * by during boot/module init.
> >+ * Should not be called from interrupt context
> >
> >
> "Must not" - it contains down() and thus can sleep.

:D Yes will replace 'should not' with 'must not' in my next iteration.

Thanks for the comments and feedback.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at