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Re: [Qemu-devel] [PATCH kernel v4 7/7] virtio-balloon: tell host vm's un

From: Dave Hansen
Subject: Re: [Qemu-devel] [PATCH kernel v4 7/7] virtio-balloon: tell host vm's unused page info
Date: Fri, 4 Nov 2016 11:10:11 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

Please squish this and patch 5 together.  It makes no sense to separate

> +static void send_unused_pages_info(struct virtio_balloon *vb,
> +                             unsigned long req_id)
> +{
> +     struct scatterlist sg_in;
> +     unsigned long pfn = 0, bmap_len, pfn_limit, last_pfn, nr_pfn;
> +     struct virtqueue *vq = vb->req_vq;
> +     struct virtio_balloon_resp_hdr *hdr = vb->resp_hdr;
> +     int ret = 1, used_nr_bmap = 0, i;
> +
> +     if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP) &&
> +             vb->nr_page_bmap == 1)
> +             extend_page_bitmap(vb);
> +
> +     pfn_limit = PFNS_PER_BMAP * vb->nr_page_bmap;
> +     mutex_lock(&vb->balloon_lock);
> +     last_pfn = get_max_pfn();
> +
> +     while (ret) {
> +             clear_page_bitmap(vb);
> +             ret = get_unused_pages(pfn, pfn + pfn_limit, vb->page_bitmap,
> +                      PFNS_PER_BMAP, vb->nr_page_bmap);

This changed the underlying data structure without changing the way that
the structure is populated.

This algorithm picks a "PFNS_PER_BMAP * vb->nr_page_bmap"-sized set of
pfns, allocates a bitmap for them, the loops through all zones looking
for pages in any free list that are in that range.

Unpacking all the indirection, it looks like this:

for (pfn = 0; pfn < get_max_pfn(); pfn += BITMAP_SIZE_IN_PFNS)
                for_each_migratetype_order(order, t)
                        list_for_each(..., &zone->free_area[order])...

Let's say we do a 32k bitmap that can hold ~1M pages.  That's 4GB of
RAM.  On a 1TB system, that's 256 passes through the top-level loop.
The bottom-level lists have tens of thousands of pages in them, even on
my laptop.  Only 1/256 of these pages will get consumed in a given pass.

That's an awfully inefficient way of doing it.  This patch essentially
changed the data structure without changing the algorithm to populate it.

Please change the *algorithm* to use the new data structure efficiently.
 Such a change would only do a single pass through each freelist, and
would choose whether to use the extent-based (pfn -> range) or
bitmap-based approach based on the contents of the free lists.

You should not be using get_max_pfn().  Any patch set that continues to
use it is not likely to be using a proper algorithm.

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