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

[Xiang Zheng]

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.

I am confused that why you think it will impact the system performance? Did I
neglect something?

>>> I guess the best case for your patch is many guests with relatively
>>> small RAM sizes.
> 
> .
> 
-- 

Thanks,
Xiang