On 06/28/2013 11:48 PM, Dave Hansen wrote:
> On 06/27/2013 10:47 PM, Zheng Liu wrote:
>>> I've been doing some testing involving large amounts of
>>> page cache. It's quite painful to get hundreds of GB
>>> of page cache mapped in, especially when I am trying to
>>> do it in parallel threads. This is true even when the
>>> page cache is already allocated and I only need to map
>>> it in. The test:
>>>
>>> 1. take 160 16MB files
>>> 2. clone 160 threads, mmap the 16MB files, and either
>>> a. walk through the file touching each page
>>
>> Why not change MAP_POPULATE flag in mmap(2)? Now it is only for private
>> mappings. But maybe we could let it support shared mapping.
>
> Adding that support to mmap() will certainly _help_ some folks. But,
> anything that mmap()s something is taking mmap_sem for write. That
> means that threaded apps doing mmap()/munmap() frequently are _not_
> scalable.
>
> IOW, a process needing to do a bunch of MAP_POPULATEs isn't
> parallelizable, but one using this mechanism would be.
I look at the code, and it seems that we will handle MAP_POPULATE flag
after we release mmap_sem locking in vm_mmap_pgoff():
down_write(&mm->mmap_sem);
ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
&populate);
up_write(&mm->mmap_sem);
if (populate)
mm_populate(ret, populate);
Am I missing something?
Regards,
- Zheng
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