Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Aug 7, 2013 at 6:40 AM, Jan Kara <jack@suse.cz> wrote:
> On Mon 05-08-13 12:43:58, Andy Lutomirski wrote:
>> My application fallocates and mmaps (shared, writable) a lot (several
>> GB) of data at startup.  Those mappings are mlocked, and they live on
>> ext4.  The first write to any given page is slow because
>> ext4_da_get_block_prep can block.  This means that, to get decent
>> performance, I need to write something to all of these pages at
>> startup.  This, in turn, causes a giant IO storm as several GB of
>> zeros get pointlessly written to disk.
>>
>> This series is an attempt to add madvise(..., MADV_WILLWRITE) to
>> signal to the kernel that I will eventually write to the referenced
>> pages.  It should cause any expensive operations that happen on the
>> first write to happen immediately, but it should not result in
>> dirtying the pages.
>>
>> madvice(addr, len, MADV_WILLWRITE) returns the number of bytes that
>> the operation succeeded on or a negative error code if there was an
>> actual failure.  A return value of zero signifies that the kernel
>> doesn't know how to "willwrite" the range and that userspace should
>> implement a fallback.
>>
>> For now, it only works on shared writable ext4 mappings.  Eventually
>> it should support other filesystems as well as private pages (it
>> should COW the pages but not cause swap IO) and anonymous pages (it
>> should COW the zero page if applicable).
>>
>> The implementation leaves much to be desired.  In particular, it
>> generates dirty buffer heads on a clean page, and this scares me.
>>
>> Thoughts?
>   One question before I look at the patches: Why don't you use fallocate()
> in your application? The functionality you require seems to be pretty
> similar to it - writing to an already allocated block is usually quick.

I do use fallocate, and, IIRC, the problem was worse before I added
the fallocate call.

This could be argued to be a filesystem problem -- perhaps
page_mkwrite should never block.  I don't expect that to be fixed any
time soon (if ever).

--Andy

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