[Xen-devel] Re: Tmem vs order>0 allocation, workaround RFC

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 15/02/2010 14:31, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Good point.  BUT... do you know of any other asymmetric
> allocs/frees?  Since the 2MB allocation does fall back
> if it fails (to 4K I think?, if the patch is modified
> to restrict the "zone" to order>0&&order<9 will that
> be sufficient?

Even though that one can fall back, the point is that even one asymmetric
alloc/free (and that is by far going to be the most common one) can hoover
up the 1% 'pool' and fragment it, so that allocations that cannot fall back
can no longer use the pool.

> I know this is quite a hack...  I don't like it much
> either.  But I expect the process of restructuring all
> data structures to limit them to order==0 to take a long
> time with an even longer bug tail (AND be a whack-a-mole
> game in the future unless we disallow order>0 entirely).
> In that light (and with the low impact of this workaround),
> this hack may be just fine for a while.

Well, I think it's not only not very nice but also dubious whether it will
work in practice very well.

 -- Keir



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