[RFC v3] fiber: Increase default stack size

[Cyrill Gorcunov]

On Tue, Feb 26, 2019 at 01:26:56PM +0300, Vladimir Davydov wrote:
...
> 
> Talked to Kostja and Georgy. We agreed on the following points:
> 
>  - Regarding slab poisoning. We don't want to implement ad hoc allocator
>    for fiber stacks, neither do we really want to patch the small lib
>    for now. Let's use madvise(MADV_DONTNEED) for all fiber stacks
>    unconditionally on fiber creation. This should be okay from
>    performance point of view, because once a fiber is created, it's
>    never destroyed - it stays on the dead list until recycled.

OK

>  - 1 MB for max stack size seems to be a bit of an overkill for now.
>    The default value should be set to 256 KB, but we do need a
>    configuration option for it. Let's add it to the fiber Lua module.
>    May be done in a separate patch, but should be submitted together in
>    the same patch set.

Wait, first fiber for main cord is created before lua init, isn't it?
I already though about using lua config for it but fiber init'ed at
very early stage.

>  - 16 byte unique identifier for detecting stack overflow doesn't seem
>    to be enough. Imagine a PATH_MAX buffer allocated on stack that uses
>    only a hundred bytes for path formatting. It can easily jump over the
>    watermark. We should probably use random poisoning: say, 4 unique
>    identifiers 8 bytes each scattered a few hundred bytes apart. Some
>    math/reasoning behind this would be nice to see in the comments.

If we want to scatter we should simply put marks at page bounds.
Dirtifying somewhere inside middle of a page is useless.

>  - Since madvise() is somewhat expensive to be called on each fiber
>    recycle, we need to make the watermark dynamic. That is, keep track
>    of the number of fibers that have exceeded the watermark and when
>    there are too many of those, increase the watermark value. We could
>    probably use a histogram for this. This would allow us to decrease
>    the default stack allocation size from 64 KB down to 16 KB, which
>    would be really nice. This should be done in a separate patch, but
>    again in the scope of this issue.

Nod.

> 
>  - We definitely need this patch (or patches) to be covered with unit
>    tests. Please add corresponding cases to test/unit/fiber.cc.

Yes, need to address this too. Thanks a huge for comments!

	Cyrill