Re: [Qemu-devel] [PATCH] pflash: Only read non-zero parts of backend image

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 13.05.2019 um 13:59 hat Markus Armbruster geschrieben:
>> Xiang Zheng <address@hidden> writes:
>> 
>> > On 2019/5/10 23:16, Markus Armbruster wrote:
>> >> Xiang Zheng <address@hidden> writes:
>> >> 
>> >>> On 2019/5/9 19:59, Markus Armbruster wrote:
>> >>>> Xiang Zheng <address@hidden> writes:
>> >>>>
>> >>>>> On 2019/5/8 21:20, Markus Armbruster wrote:
>> >>>>>> Laszlo Ersek <address@hidden> 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 <address@hidden> 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.
>
> Is this discussion really a good use of our time?
>
> The patch is simple, and a few obvious improvements it brings were
> mentioned (even by yourself), such as avoiding OOM without swap; and
> with swap enabled, saving swap space for more useful content and
> saving unnecessary I/O related to accessing swap needlessly.
>
> You may consider these improvements neglegible, but even small
> improvments can add up. If you really want to measure them, it should be
> clear how to do it. I don't see value in actually setting up such
> environments just to get some numbers that show what we already know.
>
> So, what's the downside of the patch? The worst case is, the memory
> usage numbers only look better, but most people don't have a use case
> where the improvement matters. There might be some maintenance cost
> associated with the code, but it's small and I suspect this discussion
> has already cost us more time than maintaining the code will ever cost
> us.
>
> So why not just take it?

As is, the patch's commit message fails to meet the standards I set as a
maintainer, because (1) it's too vague on what the patch does, and what
its limitations are (relies on well-behaved guests), and (2) it fails to
make the case for the patch.

Fortunately, I'm not the maintainer here, Philippe is.  My standards do
not matter.