Re: fc3 and stacks

[Eric Sandeen <sandeen@xxxxxxx>]

Robin Humble wrote:

> Some ways forward:
>  - convince RedHat to put 8k stacks (or at least the option for it) and
>    XFS into AS4.

FWIW, I try to steer the conversation away from "xfs is broken with 4k 
stacks" and towards "4k stacks don't work for some applications."  :)

XFS can chew through stack a bit faster than some of the filesystems, to 
be sure.  But I've always meant to try to put together some diabolical 
"supported" config on RHEL4 and file a bug - say nfs over deeply nested 
volume managers, or something like that.

>  - try to make XFS play nicer with 4k stacks
>  - run fc3. fc3 and AS4 are very similar (~ 50% of userland rpms are
>    identical, the rest seem to be fairly minor variations), so if your
>    vendor supports AS4 then it'll almost certainly run fine on fc3

but both fc3 and RHEL4 have 4k stacksm though...

>  - install AS4 but run a fc3 (recompiled for 8k stacks) kernel

...ah, yes.

>  - install AS4 but run a stock kernel.org (or XFS cvs) kernel

Seems like with RHEL3, stock kernel.org kernels don't play nicely.  I 
hope this isn't the case with RHEL4, but I have not tested it.

But rebuilding the stock RHEL4 kernel, with tweaks to turn xfs back on, 
maybe update xfs, and re-enable 8k stacks should be possible, if not 
supported by RH. :)  Or as suggested by Andi, just pick one of the 
arches that have larger stack space (x86_64) (hm, I should verify that 
that's still fine on RH kernels... maybe they made them 1k stacks) :)

> The first 2 of these are hard, the last 2 are pretty easy but may not
> help you from a vendor support point of view. We'll probably choose the
> last option.
> The middle option (fc3) is what you are doing now, so you can get some
> idea of XFS stability with 4k stacks for your particular workload.
>
> I wonder why RedHat was so keen to remove 8k as an option. Things would
> be fairly simple if they hadn't done that.

I think there are some applications (java?) that can benefit from 
smaller stacks in terms of overal thread count & memory usage...  but I 
don't know why they went so far as to actually remove the option.

-Eric