Re: [PATCH 2/2] lib/core/fiber: Allow to extend default stack size

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Wed, Mar 27, 2019 at 12:46:18PM +0300, Cyrill Gorcunov wrote:
> On Wed, Mar 27, 2019 at 12:35:06PM +0300, Vladimir Davydov wrote:
> > > diff --git a/src/lib/core/fiber.c b/src/lib/core/fiber.c
> > > index 922a0bfe8..d3cb18a15 100644
> > > --- a/src/lib/core/fiber.c
> > > +++ b/src/lib/core/fiber.c
> > > @@ -107,18 +107,25 @@ pthread_t main_thread_id;
> > >  static size_t page_size;
> > >  static bool stack_growsdown;
> > >  
> > > +/** Indices in fiber stack sizes (FSS) */
> > >  enum {
> > > +	FSS_IDX_MINIMAL,
> > > +	FSS_IDX_WATERMARK,
> > > +	FSS_IDX_DEFAULT,
> > > +};
> > > +
> > > +/** Fiber stack sizes */
> > > +static size_t fiber_stack_size[] = {
> > >  	/* The minimum allowable fiber stack size in bytes */
> > > -	FIBER_STACK_SIZE_MINIMAL = 16384,
> > > +	[FSS_IDX_MINIMAL]	= 16384,
> > > +	/* Stack size watermark in bytes */
> > > +	[FSS_IDX_WATERMARK]	= 65536,
> > >  	/* Default fiber stack size in bytes */
> > > -	FIBER_STACK_SIZE_DEFAULT = 524288,
> > > -	/* Stack size watermark in bytes. */
> > > -	FIBER_STACK_SIZE_WATERMARK = 65536,
> > > +	[FSS_IDX_DEFAULT]	= 524288,
> > >  };
> > 
> > Please avoid unrelated changes. If you want to rename or refactor
> > something, do it in a separate patch to ease review.
> > 
> > Anyway, I don't see much point in this change - in Tarantool code it's
> > common to use a enum for storing constants.
> 
> These are not constants anymore since we're to ajust the values.

Ah, I see. Anyway, I'd leave the constants alone and instead introduce a
separate static variable to store the current stack size in, because
mixing constants (min_size, watermark) and variables (current_default)
in one array doesn't look good to me.

> 
> > > +/**
> > > + * Initialize early parameters of the stack
> > > + * and setup default variables.
> > > + */
> > > +static void
> > > +fiber_stack_init(void)
> > > +{
> > > +	static const char default_size_name[] = "FIBER_STACK_SIZE_DEFAULT";
> > 
> > There's a risk it may conflict with another environment variable.
> > At the very least, we should prefix it with TARANTOOL_.
> > 
> > However, I don't think that using an environmental variable is a proper
> > way to go. After all, Tarantool is an application server so we typically
> > set system variables right from Lua code.
> > 
> > Can we introduce a function in the fiber module for this? The tricky
> > part is that it should change the stack size of the calling fiber,
> > I guess.
> 
> Yes, stacks are allocated earlier, and we simply can't adjust already
> allocated stack thus we might need to distinguish stack sizes between
> cord_main and new fibers. This gonna be hard and I would like to escape
> such complexity. Seriously, we should use a hard rule: every stack in
> the engine has the same size allocated.
> 
> Lets think -- is there a chance we will have to provide more configurable
> settings for early init stage in future? If yes then we should invent
> early init stage for lua (or say some json fine in /etc). If no and
> there is no chance we might need something else in future, better to
> stick with simplier solution as environment variables.

Makes sense. But as this is a change in the API, we should consult
Kostja (Cc-ed).