Re: [RFC 09/15] PM / Hibernate: user, implement user_ops writer

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Tuesday 30 March 2010, Nigel Cunningham wrote:
> Hi.
> 
> On 30/03/10 09:09, Rafael J. Wysocki wrote:
> > On Monday 29 March 2010, Jiri Slaby wrote:
... 
> Okay. So the read or write terminology is from userspace's perspective, 
> right?

Yes.

> Perhaps that's part of what I've found confusing about the kernel side of things.
> 
> > [Note that snapshot_[read|write]_next() belong to the memory management
> > part of the hibernation subsystem and that's exactly why they are in
> > snapshot.c.]
> >
> > Now, compression can happen in two places: while the image is created
> > or after it has been created (current behavior).  In the latter case, the image
> > pages need not be compressed in place, they may be compressed after being
> > returned by snapshot_read_next(), in a temporary buffer (that's now s2disk
> > does that).  So you can arrange things like this:
> >
> > create image
> > repeat:
> > - snapshot_read_next() ->  buffer
> > - if buffer is full, compress it (possibly encrypt it) and write the result to
> >    the storage
> >
> > This way you'd just avoid all of the complications and I fail to see any
> > drawbacks.
> >
> > Now, an attractive thing would be to compress data while creating the image
> > and that may be done in the following way:
> >
> > have a buffer ready
> > repeat:
> > - copy image pages to the buffer (instead of copying them directly into the
> >    image storage space)
> > - if buffer is full, compress it and copy the result to the image storage
> >    space, page by page.
> 
> A few points that might be worth considering:
> 
> Wouldn't compressing the image while creating it rather than while 
> writing increase the overall time taken to hibernate (since the time 
> taken can't then be combined with the time for writing the image)?

It would, but that's attractive anyway, because the image could be larger than
1/2 of memory this way without using the LRU pages as temporary storage
space (which I admit I'm reluctant to do).

> Wouldn't it also increase the memory requirements?

Not really, or just a little bit (the size of the buffer).  I'm talking about
the image that's created atomically after we've frozen devices.

Rafael