Re: [Qemu-devel] Migration dirty bitmap: should only mark pages as dirty after they have been sent

["Li, Liang Z" <liang.z.li@intel.com>]

> > > > > > > > > I think this is "very" wasteful. Assume the workload
> > > > > > > > > writes the pages
> > > > > dirty randomly within the guest address space, and the transfer
> > > > > speed is constant. Intuitively, I think nearly half of the dirty
> > > > > pages produced in Iteration 1 is not really dirty. This means
> > > > > the time of Iteration 2 is double of that to send only really dirty pages.
> > > > > > > >
> > > > > > > > It makes sense, can you get some perf numbers to show what
> > > > > > > > kinds of workloads get impacted the most?  That would also
> > > > > > > > help us to figure out what kinds of speed improvements we
> > > > > > > > can
> > > expect.
> > > > > > > >
> > > > > > > >
> > > > > > > > 		Amit
> > > > > > >
> > > > > > > I have picked up 6 workloads and got the following
> > > > > > > statistics numbers of every iteration (except the last
> > > > > > > stop-copy one) during
> > > precopy.
> > > > > > > These numbers are obtained with the basic precopy migration,
> > > > > > > without the capabilities like xbzrle or compression, etc.
> > > > > > > The network for the migration is exclusive, with a separate
> > > > > > > network for
> > > the workloads.
> > > > > > > They are both gigabit ethernet. I use qemu-2.5.1.
> > > > > > >
> > > > > > > Three (booting, idle, web server) of them converged to the
> > > > > > > stop-copy
> > > > > phase,
> > > > > > > with the given bandwidth and default downtime (300ms), while
> > > > > > > the other three (kernel compilation, zeusmp, memcached) did not.
> > > > > > >
> > > > > > > One page is "not-really-dirty", if it is written first and
> > > > > > > is sent later (and not written again after that) during one
> > > > > > > iteration. I guess this would not happen so often during the
> > > > > > > other iterations as during the 1st iteration. Because all
> > > > > > > the pages of the VM are sent to the dest node
> > > > > during
> > > > > > > the 1st iteration, while during the others, only part of the
> > > > > > > pages are
> > > sent.
> > > > > > > So I think the "not-really-dirty" pages should be produced
> > > > > > > mainly during the 1st iteration , and maybe very little
> > > > > > > during the other
> > > iterations.
> > > > > > >
> > > > > > > If we could avoid resending the "not-really-dirty" pages,
> > > > > > > intuitively, I think the time spent on Iteration 2 would be
> > > > > > > halved. This is a chain
> > > > > reaction,
> > > > > > > because the dirty pages produced during Iteration 2 is
> > > > > > > halved, which
> > > > > incurs
> > > > > > > that the time spent on Iteration 3 is halved, then Iteration 4, 5...
> > > > > >
> > > > > > Yes; these numbers don't show how many of them are false dirty
> > > though.
> > > > > >
> > > > > > One problem is thinking about pages that have been redirtied,
> > > > > > if the page is
> > > > > dirtied
> > > > > > after the sync but before the network write then it's the
> > > > > > false-dirty that you're describing.
> > > > > >
> > > > > > However, if the page is being written a few times, and so it
> > > > > > would have
> > > > > been written
> > > > > > after the network write then it isn't a false-dirty.
> > > > > >
> > > > > > You might be able to figure that out with some kernel tracing
> > > > > > of when the
> > > > > dirtying
> > > > > > happens, but it might be easier to write the fix!
> > > > > >
> > > > > > Dave
> > > > >
> > > > > Hi, I have made some new progress now.
> > > > >
> > > > > To tell how many false dirty pages there are exactly in each
> > > > > iteration, I malloc a buffer in memory as big as the size of the
> > > > > whole VM memory. When a page is transferred to the dest node, it
> > > > > is copied to the buffer; During the next iteration, if one page
> > > > > is transferred, it is compared to the old one in the buffer, and
> > > > > the old one will be replaced for next comparison if it is really dirty.
> > > > > Thus, we are now able to get the exact number of false dirty pages.
> > > > >
> > > > > This time, I use 15 workloads to get the statistic number. They are:
> > > > >
> > > > >   1. 11 benchmarks picked up from cpu2006 benchmark suit. They
> > > > > are all scientific
> > > > >      computing workloads like Quantum Chromodynamics, Fluid
> > > > > Dynamics,
> > > etc.
> > > > > I pick
> > > > >      up these 11 benchmarks because compared to others, they
> > > > > have bigger memory
> > > > >      occupation and higher memory dirty rate. Thus most of them
> > > > > could not converge
> > > > >      to stop-and-copy using the default migration speed (32MB/s).
> > > > >   2. kernel compilation
> > > > >   3. idle VM
> > > > >   4. Apache web server which serves static content
> > > > >
> > > > >   (the above workloads are all running in VM with 1 vcpu and 1GB
> > > > > memory, and the
> > > > >    migration speed is the default 32MB/s)
> > > > >
> > > > >   5. Memcached. The VM has 6 cpu cores and 6GB memory, and 4GB
> > > > > are used as the cache.
> > > > >      After filling up the 4GB cache, a client writes the cache
> > > > > at a constant
> > > speed
> > > > >      during migration. This time, migration speed has no limit,
> > > > > and is up to
> > > the
> > > > >      capability of 1Gbps Ethernet.
> > > > >
> > > > > Summarize the results first: (and you can read the precise
> > > > > number
> > > > > below)
> > > > >
> > > > >   1. 4 of these 15 workloads have a big proportion (>60%, even
> > > > > >80% during some iterations)
> > > > >      of false dirty pages out of all the dirty pages since
> > > > > iteration 2 (and the
> > > big
> > > > >      proportion lasts during the following iterations). They are
> > > cpu2006.zeusmp,
> > > > >      cpu2006.bzip2, cpu2006.mcf, and memcached.
> > > > >   2. 2 workloads (idle, webserver) spend most of the migration
> > > > > time on iteration 1, even
> > > > >      though the proportion of false dirty pages is big since
> > > > > iteration 2, the space to
> > > > >      optimize is small.
> > > > >   3. 1 workload (kernel compilation) only have a big proportion
> > > > > during iteration 2, not
> > > > >      in the other iterations.
> > > > >   4. 8 workloads (the other 8 benchmarks of cpu2006) have little
> > > > > proportion of false
> > > > >      dirty pages since iteration 2. So the spaces to optimize
> > > > > for them are
> > > small.
> > > > >
> > > > > Now I want to talk a little more about the reasons why false
> > > > > dirty pages are produced.
> > > > > The first reason is what we have discussed before---the
> > > > > mechanism to track the dirty pages.
> > > > > And then I come up with another reason. Here is the situation: a
> > > > > write operation to one memory page happens, but it doesn't
> > > > > change any content of the page. So it's "write but not dirty",
> > > > > and kernel still marks it as dirty. One guy in our lab has done
> > > > > some experiments to figure out the proportion of "write but not
> dirty"
> > > > > operations, and he uses the cpu2006 benchmark suit. According to
> > > > > his results, general workloads has a little proportion (<10%) of
> > > > > "write but not dirty" out of all the write operations, while few
> > > > > workloads have higher proportion (one even as high as 50%). Now
> > > > > we are not sure why "write but not dirty" would happen, it just
> happened.
> > > > >
> > > > > So these two reasons contribute to the false dirty pages. To
> > > > > optimize, I compute and store the SHA1 hash before transferring
> > > > > each page. Next time, if one page needs retransmission, its
> > > > > SHA1 hash is computed again, and compared to the old hash. If
> > > > > the hash is the same, it's a false dirty page, and we just skip
> > > > > this page; Otherwise, the page is transferred, and the new hash
> > > > > replaces the old one for next comparison.
> > > > > The reason to use SHA1 hash but not byte-by-byte comparison is
> > > > > the memory overheads. One SHA1 hash is 20 bytes. So we need
> > > > > extra
> > > > > 20/4096 (<1/200) memory space of the whole VM memory, which is
> > > > > relatively small.
> > > > > As far as I know, SHA1 hash is widely used in the scenes of
> > > > > deduplication for backup systems.
> > > > > They have proven that the probability of hash collision is far
> > > > > smaller than disk hardware fault, so it's secure hash, that is,
> > > > > if the hashes of two chunks are the same, the content must be the
> same.
> > > > > So I think the SHA1 hash could replace byte-to-byte comparison
> > > > > in the VM memory scenery.
> > > > >
> > > > > Then I do the same migration experiments using the SHA1 hash.
> > > > > For the 4 workloads which have big proportions of false dirty
> > > > > pages, the improvement is remarkable. Without optimization, they
> > > > > either can not converge to stop-and-copy, or take a very long time to
> complete.
> > > > > With the
> > > > > SHA1 hash method, all of them now complete in a relatively short
> time.
> > > > > For the reason I have talked above, the other workloads don't
> > > > > get notable improvements from the optimization. So below, I only
> > > > > show the exact number after optimization for the 4 workloads
> > > > > with remarkable improvements.
> > > > >
> > > > > Any comments or suggestions?
> > > >
> > > > Maybe you can compare the performance of your solution as that of
> > > XBZRLE to see which one is better.
> > > > The merit of using SHA1 is that it can avoid data copy as that in
> > > > XBZRLE, and
> > > need less buffer.
> > > > How about the overhead of calculating the SHA1? Is it faster than
> > > > copying a
> > > page?
> > > >
> > > > Liang
> > > >
> > > >
> > >
> > > Yes, XBZRLE is able to handle the false dirty pages. However, if we
> > > want to avoid transferring all of the false dirty pages using
> > > XBZRLE, we need a buffer as big as the whole VM memory, while SHA1
> > > needs a much small buffer. Of course, if we have a buffer as big as
> > > the whole VM memory using XBZRLE, we could transfer less data on
> > > network than SHA1, because XBZRLE is able to compress similar pages.
> > > In a word, yes, the merit of using SHA1 is that it needs much less
> > > buffer, and leads to nice improvement if there are many false dirty pages.
> > >
> >
> > The current implementation of XBZRLE begins to buffer page from the
> > second iteration, Maybe it's worth to make it start to work from the first
> iteration based on your finding.
> >
> > > In terms of the overhead of calculating the SHA1 compared with
> > > transferring a page, it's related to the CPU and network
> > > performance. In my test environment(Intel Xeon
> > > E5620 @2.4GHz, 1Gbps Ethernet), I didn't observe obvious extra
> > > computing overhead caused by calculating the SHA1, because the
> > > throughput of network (got by "info migrate") remains almost the same.
> >
> > You can check the CPU usage, or to measure the time spend on a local
> > live migration  which use SHA1/ XBZRLE.
> >
> > Liang
> >
> >
> 
> I compare SHA1 with XBZRLE. I use XBZRLE in two ways:
> 1. Begins to buffer pages from iteration 1; 2. As current implementation,
> begins to buffer pages from iteration 2.
> 
> I post the results of three workloads: cpu2006.zeusmp, cpu2006.mcf,
> memcached.
> I set the cache size as 256MB for zeusmp & mcf (they run in VM with 1GB
> ram), and set the cache size as 1GB for memcached (it run in VM with 6GB
> ram, and memcached takes 4GB as cache).
> 
> As you can read from the data below, beginning to buffer pages from
> iteration 1 is better than the current implementation(from iteration 2),
> because the total migration time is shorter.
> 
> SHA1 is better than the XBZRLE with the cache size I choose, because it leads
> to shorter migration time, and consumes far less memory overhead (<1/200
> of the total VM memory).
> 

Hi Chunguang,

Have you tried to use a large XBZRLE cache size which equals to the guest's RAM size?
Is SHA1 faster in that case?

Thanks!
Liang