Re: [PATCH] idr: Document ida tree sections

From: Kent Overstreet
Date: Tue Aug 13 2013 - 18:28:01 EST

On Tue, Aug 13, 2013 at 06:19:28PM -0400, Tejun Heo wrote:
> Hello,
> On Tue, Aug 13, 2013 at 03:13:08PM -0700, Kent Overstreet wrote:
> > If you're convinced this is a real issue though - how about
> It is a real issue. Large order allocation is fine for optimization
> but shouldn't be depended upon. It does fail easily without
> compaction and compaction is heavy-ass operation which will blow up
> any minute performance advantage you might get from avoiding proper
> radix tree implementation.
> > IDA_SECTION_SIZE conditional on CONFIG_COMPACTION, so we use order 2 or
> > 3 allocations if CONFIG_COMPACTION=n?
> >
> > Then the max size toplevel array of pointers to segments would be
> > bigger, but that's only an issue when we're allocating up to near
> > INT_MAX ids, so it's difficult to see how _that_ would be an issue on a
> > small/embedded system... and we could even use vmalloc for that
> > allocation when the size of that array is > IDA_SECTION_SIZE.
> What about cyclic allocations then? This is natrually a radix tree
> problem. I don't know why you're resisting radix tree so much here.

It's only naturally a radix tree problem _if_ you require sparseness.
Otherwise, radix trees require pointer chasing, which we can avoid -
which saves us both the cost of chasing pointers (which is significant)
and the overhead of storing them.

The patch handles cyclic allocation by limiting sparseness - we talked
about this and I thought you were ok with this solution, though it was
awhile ago and I could be misremembering your comments.

To recap, here's the code that implements that sparseness limiting, it's
documented in ida_alloc_cyclic()'s docs:

static int __ida_alloc_cyclic(struct ida *ida, unsigned start, unsigned end,
gfp_t gfp, unsigned long *flags)
int ret;
unsigned id;

ret = __ida_alloc_range_multiple(ida, &id, 1,
max(start, ida->cur_id),
end, gfp, flags);

if (ret < 0)
ret = __ida_alloc_range_multiple(ida, &id, 1, start,
end, gfp, flags);
if (ret == 1) {
ida->cur_id = id + 1;
if ((ida->cur_id - start) / 2 > max(1024U, ida->allocated_ids))
ida->cur_id = 0;

return id;

return ret;
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