[PATCH v2 0/8] Handle mmaped regions in cache [more analysis]

Atsushi Kumagai ats-kumagai at wm.jp.nec.com
Thu Mar 12 21:10:22 PDT 2015


(Note: my email address has changed.)

In x86_64, calling ioremap/iounmap per page in copy_oldmem_page()
causes big performance degradation, so mmap() was introduced on
/proc/vmcore. However, there is no big difference between read() and
mmap() in s390 since it doesn't need ioremap/iounmap in copy_oldmem_page(),
so other issues have been revealed, right?


>> I counted the mmap and read system calls with "perf stat":
>>                      mmap   unmap   read =    sum
>>   ===============================================
>>   mmap -d0            482     443    165     1090
>>   mmap -d31         13454   13414    165    27033
>>   non-mmap -d0         34       3 458917   458954
>>   non-mmap -d31        34       3  74273    74310
>If your VM has 1.5 GiB of RAM, then the numbers for -d0 look
>reasonable. For -d31, we should be able to do better than this
>by allocating more cache slots and improving the algorithm.
>I originally didn't deem it worth the effort, but seeing almost
>30 times more mmaps than with -d0 may change my mind.

Are you going to do it as v3 patch?
I'm going to release v1.5.8 soon, so I'll adopt v2 patch if
you don't think updating it.

Atsushi Kumagai

>> Here the actual results I got with "perf record":
>> $ time ./makedumpfile  -d 31 /proc/vmcore  /dev/null -f
>>   Output of "perf report" for mmap case:
>>    /* Most time spent for unmap in kernel */
>>    29.75%  makedumpfile  [kernel.kallsyms]  [k] unmap_single_vma
>>     9.84%  makedumpfile  [kernel.kallsyms]  [k] remap_pfn_range
>>     8.49%  makedumpfile  [kernel.kallsyms]  [k] vm_normal_page
>>    /* Still some mmap overhead in makedumpfile readmem() */
>>    21.56%  makedumpfile  makedumpfile       [.] readmem
>This number is interesting. Did you compile makedumpfile with
>optimizations? If yes, then this number probably includes some
>functions which were inlined.
>>     8.49%  makedumpfile  makedumpfile       [.] write_kdump_pages_cyclic
>>   Output of "perf report" for non-mmap case:
>>    /* Time spent for sys_read (that needs also two copy operations on s390 :() */
>>    25.32%  makedumpfile  [kernel.kallsyms]  [k] memcpy_real
>>    22.74%  makedumpfile  [kernel.kallsyms]  [k] __copy_to_user
>>    /* readmem() for read path is cheaper ? */
>>    13.49%  makedumpfile  makedumpfile       [.] write_kdump_pages_cyclic
>>     4.53%  makedumpfile  makedumpfile       [.] readmem
>Yes, much lower overhead of readmem is strange. For a moment I
>suspected wrong accounting of the page fault handler, but then I
>realized that for /proc/vmcore, all page table entries are created
>with the present bit set already, so there are no page faults...
>I haven't had time yet to set up a system for reproduction, but I'll
>try to identify what's eating up the CPU time in readmem().
>> I hope this analysis helps more than it confuses :-)
>> As a conclusion, we could think of mapping larger chunks
>> also for the fragmented case of -d 31 to reduce the amount
>> of mmap/munmap calls.
>I agree in general. Memory mapped through /proc/vmcore does not
>increase run-time memory requirements, because it only adds a mapping
>to the old kernel's memory. The only limiting factor is the virtual
>address space. On many architectures, this is no issue at all, and we
>could simply map the whole file at beginning. On some architectures,
>the virtual address space is smaller than possible physical RAM, so
>this approach would not work for them.
>> Another open question was why the mmap case consumes more CPU
>> time in readmem() than the read case. Our theory is that the
>> first memory access is slower because it is not in the HW
>> cache. For the mmap case userspace issues the first access (copy
>> to makdumpfile cache) and for the read case the kernel issues
>> the first access (memcpy_real/copy_to_user). Therefore the
>> cache miss is accounted to userspace for mmap and to kernel for
>> read.
>I have no idea how to measure this on s390. On x86_64 I would add some
>asm code to read TSC before and after the memory access instruction. I
>guess there is a similar counter on s390. Suggestions?
>> And last but not least, perhaps on s390 we could replace
>> the bounce buffer used for memcpy_real()/copy_to_user() by
>> some more inteligent solution.
>Which would then improve the non-mmap times even more, right?
>Petr T

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