Re: [Qemu-devel] [PATCH 1/8] migration: stop compressing page in migration thread

[Peter Xu]

On Wed, Mar 28, 2018 at 03:30:06PM +0800, Wei Wang wrote:
> On 03/27/2018 11:24 PM, Xiao Guangrong wrote:
> > 
> > 
> > On 03/28/2018 11:01 AM, Wang, Wei W wrote:
> > > On Tuesday, March 13, 2018 3:58 PM, Xiao Guangrong wrote:
> > > > 
> > > > As compression is a heavy work, do not do it in migration
> > > > thread, instead, we
> > > > post it out as a normal page
> > > > 
> > > > Signed-off-by: Xiao Guangrong <address@hidden>
> > > 
> > > 
> > > Hi Guangrong,
> > > 
> > > Dave asked me to help review your patch, so I will just drop my 2
> > > cents wherever possible, and hope that could be inspiring for your
> > > work.
> > 
> > Thank you both for the nice help on the work. :)
> > 
> > > 
> > > 
> > > > ---
> > > >   migration/ram.c | 32 ++++++++++++++++----------------
> > > >   1 file changed, 16 insertions(+), 16 deletions(-)
> > > > 
> > > > diff --git a/migration/ram.c b/migration/ram.c index
> > > > 7266351fd0..615693f180 100644
> > > > --- a/migration/ram.c
> > > > +++ b/migration/ram.c
> > > > @@ -1132,7 +1132,7 @@ static int ram_save_compressed_page(RAMState
> > > > *rs, PageSearchStatus *pss,
> > > >       int pages = -1;
> > > >       uint64_t bytes_xmit = 0;
> > > >       uint8_t *p;
> > > > -    int ret, blen;
> > > > +    int ret;
> > > >       RAMBlock *block = pss->block;
> > > >       ram_addr_t offset = pss->page << TARGET_PAGE_BITS;
> > > > 
> > > > @@ -1162,23 +1162,23 @@ static int
> > > > ram_save_compressed_page(RAMState *rs, PageSearchStatus *pss,
> > > >           if (block != rs->last_sent_block) {
> > > >               flush_compressed_data(rs);
> > > >               pages = save_zero_page(rs, block, offset);
> > > > -            if (pages == -1) {
> > > > -                /* Make sure the first page is sent out before
> > > > other pages */
> > > > -                bytes_xmit = save_page_header(rs, rs->f, block,
> > > > offset |
> > > > - RAM_SAVE_FLAG_COMPRESS_PAGE);
> > > > -                blen = qemu_put_compression_data(rs->f, p,
> > > > TARGET_PAGE_SIZE,
> > > > - migrate_compress_level());
> > > > -                if (blen > 0) {
> > > > -                    ram_counters.transferred += bytes_xmit + blen;
> > > > -                    ram_counters.normal++;
> > > > -                    pages = 1;
> > > > -                } else {
> > > > -                    qemu_file_set_error(rs->f, blen);
> > > > -                    error_report("compressed data failed!");
> > > > -                }
> > > > -            }
> > > >               if (pages > 0) {
> > > >                   ram_release_pages(block->idstr, offset, pages);
> > > > +            } else {
> > > > +                /*
> > > > +                 * Make sure the first page is sent out before
> > > > other pages.
> > > > +                 *
> > > > +                 * we post it as normal page as compression
> > > > will take much
> > > > +                 * CPU resource.
> > > > +                 */
> > > > +                ram_counters.transferred +=
> > > > save_page_header(rs, rs->f, block,
> > > > +                                                offset |
> > > > RAM_SAVE_FLAG_PAGE);
> > > > +                qemu_put_buffer_async(rs->f, p, TARGET_PAGE_SIZE,
> > > > +                                      migrate_release_ram() &
> > > > + migration_in_postcopy());
> > > > +                ram_counters.transferred += TARGET_PAGE_SIZE;
> > > > +                ram_counters.normal++;
> > > > +                pages = 1;
> > > >               }
> > > >           } else {
> > > >               pages = save_zero_page(rs, block, offset);
> > > > -- 
> > > 
> > > I agree that this patch is an improvement for the current
> > > implementation. So just pile up mine here:
> > > Reviewed-by: Wei Wang <address@hidden>
> > 
> > Thanks.
> > 
> > > 
> > > 
> > > If you are interested in something more aggressive, I can share an
> > > alternative approach, which I think would be better. Please see
> > > below.
> > > 
> > > Actually, we can use the multi-threaded compression for the first
> > > page as well, which will not block the migration thread progress.
> > > The advantage is that we can enjoy the compression benefit for the
> > > first page and meanwhile not blocking the migration thread - the
> > > page is given to a compression thread and compressed asynchronously
> > > to the migration thread execution.
> > > 
> > 
> > Yes, it is a good point.
> > 
> > > The main barrier to achieving the above that is that we need to make
> > > sure the first page of each block is sent first in the
> > > multi-threaded environment. We can twist the current implementation
> > > to achieve that, which is not hard:
> > > 
> > > For example, we can add a new flag to RAMBlock - bool
> > > first_page_added. In each thread of compression, they need
> > > 1) check if this is the first page of the block.
> > > 2) If it is the first page, set block->first_page_added after
> > > sending the page;
> > > 3) If it is not the first the page, wait to send the page only when
> > > block->first_page_added is set.
> > 
> > 
> > So there is another barrier introduced which hurts the parallel...
> > 
> > Hmm, we need more deliberate consideration on this point, let me think
> > it over after this work.
> > 
> 
> Sure. Just a reminder, this doesn't have to be a barrier to the compression,
> it is just used to serialize sending the pages.
> 
> Btw, this reminds me a possible bug in this patch (also in the current
> upstream code): there appears to be no guarantee that the first page will be
> sent before others. The migration thread and the compression thread use
> different buffers. The migration thread just puts the first page into its
> buffer first,  the second page is put to the compression thread buffer
> later. There appears to be no guarantee that the migration thread will flush
> its buffer before the compression thread.

IIUC finally the compression buffers will be queued into the migration
IO stream, so they are still serialized.

In compress_page_with_multi_thread() there is:

        bytes_xmit = qemu_put_qemu_file(rs->f, comp_param[idx].file);

comp_param[idx].file should be the compression buffer.

rs->f should be the migration IO stream. Thanks,

-- 
Peter Xu