On Tuesday, September 10, 2019 at 8:09:08 AM UTC-4, pusno...@gmail.com
wrote:
>
> I mean "allocated but not used" case.
>
> In the example, it only uses the first 4KB of 512MB. 4KB is a small
> portion of 128MB.
>
> I also totally agree with mmap-backed malloc for large memory allocation.
>
We are always on the lookout for volunteers so we would welcome a patch
implementing it :-)
>
>
> Wonsup Yoon
>
>
> 2019년 9월 10일 화요일 오후 3시 52분 52초 UTC+9, Nadav Har'El 님의 말:
>>
>>
>> On Mon, Sep 9, 2019 at 2:41 PM <pusno...@gmail.com> wrote:
>>
>>> Hi,
>>> I found malloc returns physical address in mempool area and does not
>>> perform demand paging (only mmap does).
>>> Is there any reason for the design choice?
>>>
>>
>> I guess you're not really asking about *demand* paging ("swapping")
>> because this feature is usually an unnecessary complication in
>> single-application kernels. If I understand correctly, your question more
>> about why does malloc() allocate physically contiguous memory unlike mmap().
>>
>> The answer is that we originally did this because of the issue of huge
>> pages. Modern CPUs have another level above the regular 4K pages - 2 MB
>> pages called "huge pages". Applications get a performance boost by using
>> huge pages, because the CPU's page table cache (the TLB) can only fit a
>> fixed number of pages, so an application using few huge tables instead of a
>> large number of small pages will have a higher hit rate in this cache, and
>> improved performance. So it is inefficient to allocate a 8 KB allocation
>> using small pages (two separate pages in physical pages but contiguous in
>> virtual memory) - it is more efficient to set up huge pages and return the
>> 8KB allocation as a contiguous part of such a huge-table. We measured this
>> to noticeably improve (by a few percent) of applications which use memory
>> allocated in small and-medium sized allocations.
>>
>> That being said, for really large allocations - significantly over 2MB
>> (the huge-page size) - there's no real reason why we need those to be
>> contiguous in physical memory - we can build them from 2MB huge-pages, each
>> contiguous in physical memory but overall the entire object is not. In
>> fact, this is *exactly* what our mmap() does. So it would be nice if
>> malloc() could fall back to call mmap() for allocations larger than some
>> threshold (2MB, 4MB, or whatever). This is definitely doable - we have an
>> open issue about this: https://github.com/cloudius-systems/osv/issues/854
>> - and it explains how it can be done.
>>
>>
>>> OSv fails, even if it only uses small portion of allocated memory.
>>>
>>
>> In your example, if I understand correctly, you tried to allocate 512 MB
>> with a 128 MB memory, so it's not "a small portion" of memory - it's more
>> than the memory you have :-)
>>
>> But the issue still has merit. If you tried to allocate 50 MB it might
>> have still have failed, because of memory fragmentation (i.e., we have 50
>> MB free memory, but not contiguous in physical memory).
>>
>>
>>>
>>> #include <stdio.h>
>>> #include <stdlib.h>
>>> #include <sys/mman.h>
>>>
>>> int main()
>>> {
>>> size_t size = 512 * 1024 * 1024;
>>> printf("Hello from main\n");
>>> printf("allocation %x start\n", size);
>>> //int *p = (int *)malloc(size); // FAIL
>>> int *p = (int *)mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE |
>>> MAP_ANONYMOUS, -1, 0); // OK
>>> printf("allocation %x = %p\n", size, p);
>>> *(p) = 512;
>>> printf("access done\n");
>>>
>>> return 0;
>>> }
>>>
>>>
>>> Thanks.
>>>
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>>>
>>
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