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Message-ID: <20160524181054-mutt-send-email-mst@redhat.com> Date: Tue, 24 May 2016 18:12:43 +0300 From: "Michael S. Tsirkin" <mst@...hat.com> To: "Li, Liang Z" <liang.z.li@...el.com> Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>, "virtualization@...ts.linux-foundation.org" <virtualization@...ts.linux-foundation.org>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, "dgilbert@...hat.com" <dgilbert@...hat.com>, "amit.shah@...hat.com" <amit.shah@...hat.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org> Subject: Re: [PATCH RFC kernel] balloon: speed up inflating/deflating process On Tue, May 24, 2016 at 02:36:08PM +0000, Li, Liang Z wrote: > > > > > > This can be pre-initialized, correct? > > > > > > > > > > pre-initialized? I am not quite understand your mean. > > > > > > > > I think you can maintain sg as part of device state and init sg with the > > bitmap. > > > > > > > > > > I got it. > > > > > > > > > This is grossly inefficient if you only requested a single page. > > > > > > And it's also allocating memory very aggressively without ever > > > > > > telling the host what is going on. > > > > > > > > > > If only requested a single page, there is no need to send the > > > > > entire page bitmap, This RFC patch has already considered about this. > > > > > > > > where's that addressed in code? > > > > > > > > > > By record the start_pfn and end_pfn. > > > > > > The start_pfn & end_pfn will be updated in set_page_bitmap() and will > > > be used in the function tell_host(): > > > > > > ---------------------------------------------------------------------- > > > ----------- > > > +static void set_page_bitmap(struct virtio_balloon *vb, struct page > > > +*page) { > > > + unsigned int i; > > > + unsigned long *bitmap = vb->page_bitmap; > > > + unsigned long balloon_pfn = page_to_balloon_pfn(page); > > > + > > > + for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++) > > > + set_bit(balloon_pfn + i, bitmap); > > > > BTW, there's a page size value in header so there is no longer need to set > > multiple bits per page. > > Yes, you are right. > > > > > > + if (balloon_pfn < vb->start_pfn) > > > + vb->start_pfn = balloon_pfn; > > > + if (balloon_pfn > vb->end_pfn) > > > + vb->end_pfn = balloon_pfn; > > > +} > > > > Sounds good, but you also need to limit by allocated bitmap size. > > Why should we limit the page bitmap size? Is it no good to send a large page bitmap? > or to save the memory used for page bitmap? Or some other reason? To save memory. First allocating a large bitmap can fail, second this is pinned memory that is wasted - it's unused most of the time while guest is running. > > > > > > > > + unsigned long start_pfn, end_pfn, flags = 0, bmap_len; > > > + struct scatterlist sg[5]; > > > + > > > + start_pfn = rounddown(vb->start_pfn, BITS_PER_LONG); > > > + end_pfn = roundup(vb->end_pfn, BITS_PER_LONG); > > > + bmap_len = (end_pfn - start_pfn) / BITS_PER_LONG * > > sizeof(long); > > > + > > > + sg_init_table(sg, 5); > > > + sg_set_buf(&sg[0], &flags, sizeof(flags)); > > > + sg_set_buf(&sg[1], &start_pfn, sizeof(start_pfn)); > > > + sg_set_buf(&sg[2], &page_shift, sizeof(page_shift)); > > > + sg_set_buf(&sg[3], &bmap_len, sizeof(bmap_len)); > > > + sg_set_buf(&sg[4], vb->page_bitmap + > > > + (start_pfn / BITS_PER_LONG), bmap_len); > > > > Looks wrong. start_pfn should start at offset 0 I think ... > > I don't know what is wrong here, could you tell me why? > > Thanks! > > Liang start_pfn should mean "bit 0 in bitmap refers to pfn X". So it does not make sense to also add it as offset within bitmap. -- MST
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