Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH 4/6] PM/Hibernate: Rework shrinking of memory | Date | Wed, 13 May 2009 22:55:03 +0200 |
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On Wednesday 13 May 2009, Andrew Morton wrote: > On Wed, 13 May 2009 10:39:25 +0200 > "Rafael J. Wysocki" <rjw@sisk.pl> wrote: > > > From: Rafael J. Wysocki <rjw@sisk.pl> > > > > Rework swsusp_shrink_memory() so that it calls shrink_all_memory() > > just once to make some room for the image and then allocates memory > > to apply more pressure to the memory management subsystem, if > > necessary. > > > > Unfortunately, we don't seem to be able to drop shrink_all_memory() > > entirely just yet, because that would lead to huge performance > > regressions in some test cases. > > > > Isn't this a somewhat large problem?
Yes, it is. The thing is 8 times slower (15 s vs 2 s) without the shrink_all_memory() in at least one test case. 100% reproducible.
> The main point (I thought) was to remove shrink_all_memory(). Instead, > we're retaining it and adding even more stuff?
The idea is that afterwards we can drop shrink_all_memory() once the performance problem has been resolved. Also, we now allocate memory for the image using GFP_KERNEL instead of doing it with GFP_ATOMIC after freezing devices. I'd think that's an improvement?
> > +/** > > + * compute_fraction - Compute approximate fraction x * (a/b) > > + * @x: Number to multiply. > > + * @numerator: Numerator of the fraction (a). > > + * @denominator: Denominator of the fraction (b). > > * > > - * Notice: all userland should be stopped before it is called, or > > - * livelock is possible. > > + * Compute an approximate value of the expression x * (a/b), where a is less > > + * than b, all x, a, b are unsigned longs and x * a may be greater than the > > + * maximum unsigned long. > > */ > > +static unsigned long compute_fraction( > > + unsigned long x, unsigned long numerator, unsigned long denominator) > > I can't say I'm a great fan of the code layout here. > > static unsigned long compute_fraction(unsigned long x, unsigned long numerator, unsigned long denominator) > > or > > static unsigned long compute_fraction(unsigned long x, unsigned long numerator, > unsigned long denominator) > > would be more typical.
OK
> > +{ > > + unsigned long ratio = (numerator << FRACTION_SHIFT) / denominator; > > > > -#define SHRINK_BITE 10000 > > -static inline unsigned long __shrink_memory(long tmp) > > + x *= ratio; > > + return x >> FRACTION_SHIFT; > > +} > > Strange function. Would it not be simpler/clearer to do it with 64-bit > scalars, multiplication and do_div()?
Sure, I can do it this way too. Is it fine to use u64 for this purpose?
> > +static unsigned long highmem_size( > > + unsigned long size, unsigned long highmem, unsigned long count) > > +{ > > + return highmem > count / 2 ? > > + compute_fraction(size, highmem, count) : > > + size - compute_fraction(size, count - highmem, count); > > +} > > This would be considerably easier to follow if we know what the three > arguments represent. Amount of memory? In what units? `count' of > what? > > The `count/2' thing there is quite mysterious. > > <does some reverse-engineering> > > OK, `count' is "the number of pageframes we can use". (I don't think I > helped myself a lot there). But what's up with that divde-by-two? > > <considers poking at callers to work out what `size' is> > > <gives up> > > Is this code as clear as we can possibly make it??
Heh
OK, I'll do my best to clean it up.
> > +#else > > +static inline unsigned long preallocate_image_highmem(unsigned long nr_pages) > > +{ > > + return 0; > > +} > > + > > +static inline unsigned long highmem_size( > > + unsigned long size, unsigned long highmem, unsigned long count) > > { > > - if (tmp > SHRINK_BITE) > > - tmp = SHRINK_BITE; > > - return shrink_all_memory(tmp); > > + return 0; > > } > > +#endif /* CONFIG_HIGHMEM */ > > > > +/** > > + * swsusp_shrink_memory - Make the kernel release as much memory as needed > > + * > > + * To create a hibernation image it is necessary to make a copy of every page > > + * frame in use. We also need a number of page frames to be free during > > + * hibernation for allocations made while saving the image and for device > > + * drivers, in case they need to allocate memory from their hibernation > > + * callbacks (these two numbers are given by PAGES_FOR_IO and SPARE_PAGES, > > + * respectively, both of which are rough estimates). To make this happen, we > > + * compute the total number of available page frames and allocate at least > > + * > > + * ([page frames total] + PAGES_FOR_IO + [metadata pages]) / 2 + 2 * SPARE_PAGES > > + * > > + * of them, which corresponds to the maximum size of a hibernation image. > > + * > > + * If image_size is set below the number following from the above formula, > > + * the preallocation of memory is continued until the total number of saveable > > + * pages in the system is below the requested image size or it is impossible to > > + * allocate more memory, whichever happens first. > > + */ > > OK, that helps.
Great!
Thanks for the comments. :-)
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