* Xiao Guangrong (guangrong.xiao@xxxxxxxxx) wrote: > > > On 03/26/2018 05:02 PM, Peter Xu wrote: > > On Thu, Mar 22, 2018 at 07:38:07PM +0800, Xiao Guangrong wrote: > > > > > > > > > On 03/21/2018 04:19 PM, Peter Xu wrote: > > > > On Fri, Mar 16, 2018 at 04:05:14PM +0800, Xiao Guangrong wrote: > > > > > > > > > > Hi David, > > > > > > > > > > Thanks for your review. > > > > > > > > > > On 03/15/2018 06:25 PM, Dr. David Alan Gilbert wrote: > > > > > > > > > > > > migration/ram.c | 32 ++++++++++++++++---------------- > > > > > > > > > > > > Hi, > > > > > > Do you have some performance numbers to show this helps? Were those > > > > > > taken on a normal system or were they taken with one of the compression > > > > > > accelerators (which I think the compression migration was designed for)? > > > > > > > > > > Yes, i have tested it on my desktop, i7-4790 + 16G, by locally live migrate > > > > > the VM which has 8 vCPUs + 6G memory and the max-bandwidth is limited to 350. > > > > > > > > > > During the migration, a workload which has 8 threads repeatedly written total > > > > > 6G memory in the VM. Before this patchset, its bandwidth is ~25 mbps, after > > > > > applying, the bandwidth is ~50 mbps. > > > > > > > > Hi, Guangrong, > > > > > > > > Not really review comments, but I got some questions. :) > > > > > > Your comments are always valuable to me! :) > > > > > > > > > > > IIUC this patch will only change the behavior when last_sent_block > > > > changed. I see that the performance is doubled after the change, > > > > which is really promising. However I don't fully understand why it > > > > brings such a big difference considering that IMHO current code is > > > > sending dirty pages per-RAMBlock. I mean, IMHO last_sent_block should > > > > not change frequently? Or am I wrong? > > > > > > It's depends on the configuration, each memory-region which is ram or > > > file backend has a RAMBlock. > > > > > > Actually, more benefits comes from the fact that the performance & throughput > > > of the multithreads has been improved as the threads is fed by the > > > migration thread and the result is consumed by the migration > > > thread. > > > > I'm not sure whether I got your points - I think you mean that the > > compression threads and the migration thread can form a better > > pipeline if the migration thread does not do any compression at all. > > > > I think I agree with that. > > > > However it does not really explain to me on why a very rare event > > (sending the first page of a RAMBlock, considering bitmap sync is > > rare) can greatly affect the performance (it shows a doubled boost). > > > > I understand it is trick indeed, but it is not very hard to explain. > Multi-threads (using 8 CPUs in our test) keep idle for a long time > for the origin code, however, after our patch, as the normal is > posted out async-ly that it's extremely fast as you said (the network > is almost idle for current implementation) so it has a long time that > the CPUs can be used effectively to generate more compressed data than > before. One thing to try, to explain Peter's worry, would be, for testing, to add a counter to see how often this case triggers, and perhaps add some debug to see when; Peter's right that flipping between the RAMBlocks seems odd, unless you're either doing lots of iterations or have lots of separate RAMBlocks for some reason. Dave > > Btw, about the numbers: IMHO the numbers might not be really "true > > numbers". Or say, even the bandwidth is doubled, IMHO it does not > > mean the performance is doubled. Becasue the data has changed. > > > > Previously there were only compressed pages, and now for each cycle of > > RAMBlock looping we'll send a normal page (then we'll get more thing > > to send). So IMHO we don't really know whether we sent more pages > > with this patch, we can only know we sent more bytes (e.g., an extreme > > case is that the extra 25Mbps/s are all caused by those normal pages, > > and we can be sending exactly the same number of pages like before, or > > even worse?). > > > > Current implementation uses CPU very ineffectively (it's our next work > to be posted out) that the network is almost idle so posting more data > out is a better choice，further more, migration thread plays a role for > parallel, it'd better to make it fast. > > > > > > > > > > > > Another follow-up question would be: have you measured how long time > > > > needed to compress a 4k page, and how many time to send it? I think > > > > "sending the page" is not really meaningful considering that we just > > > > put a page into the buffer (which should be extremely fast since we > > > > don't really flush it every time), however I would be curious on how > > > > slow would compressing a page be. > > > > > > I haven't benchmark the performance of zlib, i think it is CPU intensive > > > workload, particularly, there no compression-accelerator (e.g, QAT) on > > > our production. BTW, we were using lzo instead of zlib which worked > > > better for some workload. > > > > Never mind. Good to know about that. > > > > > > > > Putting a page into buffer should depend on the network, i,e, if the > > > network is congested it should take long time. :) > > > > Again, considering that I don't know much on compression (especially I > > hardly used that) mine are only questions, which should not block your > > patches to be either queued/merged/reposted when proper. :) > > Yes, i see. The discussion can potentially raise a better solution. > > Thanks for your comment, Peter! -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
