> > > > > > > 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
