Re: [RFC][PATCH 6/6] PM/Hibernate: Do not try to allocate too much memory too hard

[Pavel Machek <pavel@ucw.cz>]

Hi!

> > > We want to avoid attempting to free too much memory too hard during
> > > hibernation, so estimate the minimum size of the image to use as the
> > > lower limit for preallocating memory.
> > 
> > Why? Is freeing memory too slow?
> > 
> > It used to be that user controlled image size, so he was able to
> > balance "time to save image" vs. "responsiveness of system after
> > resume".
> > 
> > Does this just override user's preference when he chooses too small
> > image size?
> > 
> > > The approach here is based on the (experimental) observation that we
> > > can't free more page frames than the sum of:
> > > 
> > > * global_page_state(NR_SLAB_RECLAIMABLE)
> > > * global_page_state(NR_ACTIVE_ANON)
> > > * global_page_state(NR_INACTIVE_ANON)
> > > * global_page_state(NR_ACTIVE_FILE)
> > > * global_page_state(NR_INACTIVE_FILE)
> > > 
> > > and even that is usually impossible to free in practice, because some
> > > of the pages reported as global_page_state(NR_SLAB_RECLAIMABLE) can't
> > > in fact be freed.  It turns out, however, that if the sum of the
> > > above numbers is subtracted from the number of saveable pages in the
> > > system and the result is multiplied by 1.25, we get a suitable
> > > estimate of the minimum size of the image.
...
> > >  /**
> > > + * minimum_image_size - Estimate the minimum acceptable size of an image
> > > + * @saveable: The total number of saveable pages in the system.
> > > + *
> > > + * We want to avoid attempting to free too much memory too hard, so estimate the
> > > + * minimum acceptable size of a hibernation image to use as the lower limit for
> > > + * preallocating memory.
> > 
> > I don't get it. If user sets image size as 0, we should free as much
> > memory as we can. I just don't see why "we want to avoid... it".
> 
> The "as much memory as we can" is not well defined.

Well, while (1) kmalloc(1024, GFP_KERNEL | GFP_NO_OOMKILL); is
basically "as much memory as we can". I believe it is pretty well defined.

> Patches [4/6] and [5/6] make hibernation use memory allocations to force some
> memory to be freed.  However, it is not really reasonable to try to allocate
> until the allocation fails, because that stresses the memory management
> subsystem too much.  It is better to predict when it fails and stop allocating
> at that point, which is what the patch does.

Why is it wrong to stress memory management? It is a computer; it can
handle it. Does it take too long? Should the user just set image_size
higher in such case?

> The prediction is not very precise, but I think it need not be.  Even if it
> leaves a few pages more in memory, that won't be a disaster.

Well, on 128MB machine, you'll fail suspend even if it would fit if
code tried little harder...?
									Pavel
-- 
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