On Thu, 9 Mar 2006, Martin J. Bligh wrote:
> >
> > DEBUG_PAGEALLOC in particular is *fantastic* at making bugs hide.
> > I've lost many an hour trying to pin bugs down due to that.
>
> Is this backwards? We're saying DEBUG_PAGEALLOC is bad?
DEBUG_PAGEALLOC is great for finding the really stupid kinds of bugs, and
it's definitely worth doing every once in a while.
However, DEBUG_PAGEALLOC makes many things orders of magnitude slower, and
it eats memory like mad (because it turns some slabs into whole pages -
but it still doesn't help small allocation debugging that much). So unlike
DEBUG_SLAB, it's not reasonable to have it on all the time.
IOW, DEBUG_SLAB is something that a distro kernel can reasonably enable
for users by default (I think fedora-devel does, for example). In
contrast, DEBUG_PAGEALLOC is more of a "useful for special cases" thing,
where you want to validate that there's nothing _obviously_ bad going on.
> Do we NOT want to have DEBUG_SLAB and DEBUG_PAGEALLOC both enabled?
I suspect that once DEBUG_PAGEALLOC is on, whether you do DEBUG_SLAB or
not is a toss-up. The interesting cases tend to be
- neither: usable for benchmarking
- DEBUG_SLAB: perfectly usable for normal work
- DEBUG_PAGEALLOC (with or without DEBUG_SLAB): debugging tool only
At least that's my opinion, maybe others have other experiences.
Linus
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