Re: [Qemu-devel] [PATCH v8 32/54] Postcopy: Maintain sentmap and calculate discard

[Dr. David Alan Gilbert]

* Juan Quintela (address@hidden) wrote:

> > +/*
> > + * Transmit the set of pages to be discarded after precopy to the target
> > + * these are pages that:
> > + *     a) Have been previously transmitted but are now dirty again
> > + *     b) Pages that have never been transmitted, this ensures that
> > + *        any pages on the destination that have been mapped by background
> > + *        tasks get discarded (transparent huge pages is the specific 
> > concern)
> > + * Hopefully this is pretty sparse
> > + */
> > +int ram_postcopy_send_discard_bitmap(MigrationState *ms)
> > +{
> > +    int ret;
> > +
> > +    rcu_read_lock();
> > +
> > +    /* This should be our last sync, the src is now paused */
> > +    migration_bitmap_sync();
> > +
> > +    /*
> > +     * Update the sentmap to be sentmap = ~sentmap | dirty
> > +     */
> > +    bitmap_complement(ms->sentmap, ms->sentmap,
> > +               last_ram_offset() >> TARGET_PAGE_BITS);
> > +
> > +    bitmap_or(ms->sentmap, ms->sentmap, migration_bitmap,
> > +               last_ram_offset() >> TARGET_PAGE_BITS);
> 
> This bitmaps are really big, I don't know how long would take to do this
> operations here, but I think that we can avoid (at least) the
> bitmap_complement.  We can change the bitmap name to notsentbitmap, init
> it to one and clear it each time that we sent a page, no?

Done, it's now 'unsentmap' - although I suspect the complement step is
probably one of the simpler steps in the process, I'm not sure it's a vast
saving.

> We can also do the bitmap_or() at migration_sync_bitmap() time, at that
> point, we shouldn't be on the critical path?

Given that we're doing the bitmap_sync immediately before the OR, I don't
understand that line; are you talking about a modified migration_bitmap_sync?

Dave

> 
> Later, Juan.
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK