Xiang Zheng <zhengxia...@huawei.com> writes: > On 2019/5/10 23:16, Markus Armbruster wrote: >> Xiang Zheng <zhengxia...@huawei.com> writes: >> >>> On 2019/5/9 19:59, Markus Armbruster wrote: >>>> Xiang Zheng <zhengxia...@huawei.com> writes: >>>> >>>>> On 2019/5/8 21:20, Markus Armbruster wrote: >>>>>> Laszlo Ersek <ler...@redhat.com> writes: >>>>>> >>>>>>> Hi Markus, >>>>>>> >>>>>>> On 05/07/19 20:01, Markus Armbruster wrote: >>>>>>>> The subject is slightly misleading. Holes read as zero. So do >>>>>>>> non-holes full of zeroes. The patch avoids reading the former, but >>>>>>>> still reads the latter. >>>>>>>> >>>>>>>> Xiang Zheng <zhengxia...@huawei.com> writes: >>>>>>>> >>>>>>>>> Currently we fill the memory space with two 64MB NOR images when >>>>>>>>> using persistent UEFI variables on virt board. Actually we only use >>>>>>>>> a very small(non-zero) part of the memory while the rest significant >>>>>>>>> large(zero) part of memory is wasted. >>>>>>>> >>>>>>>> Neglects to mention that the "virt board" is ARM. >>>>>>>> >>>>>>>>> So this patch checks the block status and only writes the non-zero >>>>>>>>> part >>>>>>>>> into memory. This requires pflash devices to use sparse files for >>>>>>>>> backends. >>>>>>>> >>>>>>>> I started to draft an improved commit message, but then I realized this >>>>>>>> patch can't work. >>>>>>>> >>>>>>>> The pflash_cfi01 device allocates its device memory like this: >>>>>>>> >>>>>>>> memory_region_init_rom_device( >>>>>>>> &pfl->mem, OBJECT(dev), >>>>>>>> &pflash_cfi01_ops, >>>>>>>> pfl, >>>>>>>> pfl->name, total_len, &local_err); >>>>>>>> >>>>>>>> pflash_cfi02 is similar. >>>>>>>> >>>>>>>> memory_region_init_rom_device() calls >>>>>>>> memory_region_init_rom_device_nomigrate() calls qemu_ram_alloc() calls >>>>>>>> qemu_ram_alloc_internal() calls g_malloc0(). Thus, all the device >>>>>>>> memory gets written to even with this patch. >>>>>>> >>>>>>> As far as I can see, qemu_ram_alloc_internal() calls g_malloc0() only to >>>>>>> allocate the the new RAMBlock object called "new_block". The actual >>>>>>> guest RAM allocation occurs inside ram_block_add(), which is also called >>>>>>> by qemu_ram_alloc_internal(). >>>>>> >>>>>> You're right. I should've read more attentively. >>>>>> >>>>>>> One frame outwards the stack, qemu_ram_alloc() passes NULL to >>>>>>> qemu_ram_alloc_internal(), for the 4th ("host") parameter. Therefore, in >>>>>>> qemu_ram_alloc_internal(), we set "new_block->host" to NULL as well. >>>>>>> >>>>>>> Then in ram_block_add(), we take the (!new_block->host) branch, and call >>>>>>> phys_mem_alloc(). >>>>>>> >>>>>>> Unfortunately, "phys_mem_alloc" is a function pointer, set with >>>>>>> phys_mem_set_alloc(). The phys_mem_set_alloc() function is called from >>>>>>> "target/s390x/kvm.c" (setting the function pointer to >>>>>>> legacy_s390_alloc()), so it doesn't apply in this case. Therefore we end >>>>>>> up calling the default qemu_anon_ram_alloc() function, through the >>>>>>> funcptr. (I think anyway.) >>>>>>> >>>>>>> And qemu_anon_ram_alloc() boils down to mmap() + MAP_ANONYMOUS, in >>>>>>> qemu_ram_mmap(). (Even on PPC64 hosts, because qemu_anon_ram_alloc() >>>>>>> passes (-1) for "fd".) >>>>>>> >>>>>>> I may have missed something, of course -- I obviously didn't test it, >>>>>>> just speculated from the source. >>>>>> >>>>>> Thanks for your sleuthing! >>>>>> >>>>>>>> I'm afraid you neglected to test. >>>>>> >>>>>> Accusation actually unsupported. I apologize, and replace it by a >>>>>> question: have you observed the improvement you're trying to achieve, >>>>>> and if yes, how? >>>>>> >>>>> >>>>> Yes, we need to create sparse files as the backing images for pflash >>>>> device. >>>>> To create sparse files like: >>>>> >>>>> dd of="QEMU_EFI-pflash.raw" if="/dev/zero" bs=1M seek=64 count=0 >>>>> dd of="QEMU_EFI-pflash.raw" if="QEMU_EFI.fd" conv=notrunc >>>> >>>> This creates a copy of firmware binary QEMU_EFI.fd padded with a hole to >>>> 64MiB. >>>> >>>>> dd of="empty_VARS.fd" if="/dev/zero" bs=1M seek=64 count=0 >>>> >>>> This creates the varstore as a 64MiB hole. As far as I know (very >>>> little), you should use the varstore template that comes with the >>>> firmware binary. >>>> >>>> I use >>>> >>>> cp --sparse=always bld/pc-bios/edk2-arm-vars.fd . >>>> cp --sparse=always bld/pc-bios/edk2-aarch64-code.fd . >>>> >>>> These guys are already zero-padded, and I use cp to sparsify. >>>> >>>>> Start a VM with below commandline: >>>>> >>>>> -drive >>>>> file=/usr/share/edk2/aarch64/QEMU_EFI-pflash.raw,if=pflash,format=raw,unit=0,readonly=on\ >>>>> -drive >>>>> file=/usr/share/edk2/aarch64/empty_VARS.fd,if=pflash,format=raw,unit=1 \ >>>>> >>>>> Then observe the memory usage of the qemu process (THP is on). >>>>> >>>>> 1) Without this patch: >>>>> # cat /proc/`pidof qemu-system-aarch64`/smaps | grep AnonHugePages: | >>>>> grep -v ' 0 kB' >>>>> AnonHugePages: 706560 kB >>>>> AnonHugePages: 2048 kB >>>>> AnonHugePages: 65536 kB // pflash memory device >>>>> AnonHugePages: 65536 kB // pflash memory device >>>>> AnonHugePages: 2048 kB >>>>> >>>>> # ps aux | grep qemu-system-aarch64 >>>>> RSS: 879684 >>>>> >>>>> 2) After applying this patch: >>>>> # cat /proc/`pidof qemu-system-aarch64`/smaps | grep AnonHugePages: | >>>>> grep -v ' 0 kB' >>>>> AnonHugePages: 700416 kB >>>>> AnonHugePages: 2048 kB >>>>> AnonHugePages: 2048 kB // pflash memory device >>>>> AnonHugePages: 2048 kB // pflash memory device >>>>> AnonHugePages: 2048 kB >>>>> >>>>> # ps aux | grep qemu-system-aarch64 >>>>> RSS: 744380 >>>> >>>> Okay, this demonstrates the patch succeeds at mapping parts of the >>>> pflash memory as holes. >>>> >>>> Do the guests in these QEMU processes run? >>> >>> Yes. >> >> Good to know, thanks. >> >>>>> Obviously, there are at least 100MiB memory saved for each guest. >>>> >>>> For a definition of "memory". >>>> >>>> Next question: what impact on system performance do you observe? >>>> >>>> Let me explain. >>>> >>>> Virtual memory holes get filled in by demand paging on access. In other >>>> words, they remain holes only as long as nothing accesses the memory. >>>> >>>> Without your patch, we allocate pages at image read time and fill them >>>> with zeroes. If we don't access them again, the kernel will eventually >>>> page them out (assuming you're running with swap). So the steady state >>>> is "we waste some swap space", not "we waste some physical RAM". >>>> >>> >>> Not everybody wants to run with swap because it may cause low performance. >> >> Someone running without swap because he heard someone say someone said >> swap may be slow is probably throwing away performance. >> >> But I assume you mean people running without swap because they measured >> their workload and found it more performant without swap. Legitimate. > > Yes, and I had ever suffered from the high IO waits with swap.:) > >> >>>> Your patch lets us map pflash memory pages containing only zeros as >>>> holes. >>>> >>>> For pages that never get accessed, your patch avoids page allocation, >>>> filling with zeroes, writing to swap (all one-time costs), and saves >>>> some swap space (not commonly an issue). >>>> >>>> For pflash memory that gets accessed, your patch merely delays page >>>> allocation from image read time to first access. >>>> >>>> I wonder how these savings and delays affect actual system performance. >>>> Without an observable change in system performance, all we'd accomplish >>>> is changing a bunch of numers in /proc/$pid/. >>>> >>>> What improvement(s) can you observe? >>> >>> We only use pflash device for UEFI, and we hardly care about the >>> performance. >>> I think the bottleneck of the performance is the MMIO emulation, even this >>> patch would delay page allocation at the first access. >> >> I wasn't inquiring about the performance of the pflash device. I was >> inquiring about *system* performance. But let me rephrase my question. >> >> Doing work to save resources is only worthwhile if something valuable >> gets better in a measurable way. I'm asking you >> >> (1) to explain what exactly you value, and >> >> (2) to provide measurements that show improvement. >> > > What we exactly value is the cost of memory resources and it is the only > thing that this patch aims to resolve.
Then measure this cost! > I am confused that why you think it will impact the system performance? Did I > neglect something? If the patch does not impact how the system as a whole performs, then it's useless. Since you find it useful, it must have some valuable[*] observable effect for you. Tell us about it! I keep asking not to torment you, but to guide you towards building a compelling justification for your patch. However, I can only show you the path; the walking you'll have to do yourself. >>>> I guess the best case for your patch is many guests with relatively >>>> small RAM sizes. >> >> . >> [*] Changing a bunch of numbers in /proc is not valuable.