Re: ext4 DIO read performance issue on SSD

[Jiaying Zhang <jiayingz@google.com>]

On Wed, Oct 14, 2009 at 1:57 PM, Mingming <cmm@us.ibm.com> wrote:
> On Wed, 2009-10-14 at 12:48 -0700, Jiaying Zhang wrote:
>> On Wed, Oct 14, 2009 at 11:48 AM, Mingming <cmm@us.ibm.com> wrote:
>> > On Fri, 2009-10-09 at 16:34 -0700, Jiaying Zhang wrote:
>> >> Hello,
>> >>
>> >> Recently, we are evaluating the ext4 performance on a high speed SSD.
>> >> One problem we found is that ext4 performance doesn't scale well with
>> >> multiple threads or multiple AIOs reading a single file with O_DIRECT.
>> >> E.g., with 4k block size, multiple-thread DIO AIO random read on ext4
>> >> can lose up to 50% throughput compared to the results we get via RAW IO.
>> >>
>> >> After some initial analysis, we think the ext4 performance problem is caused
>> >> by the use of i_mutex lock during DIO read. I.e., during DIO read, we grab
>> >> the i_mutex lock in __blockdev_direct_IO because ext4 uses the default
>> >> DIO_LOCKING from the generic fs code. I did a quick test by calling
>> >> blockdev_direct_IO_no_locking() in ext4_direct_IO() and I saw ext4 DIO read
>> >> got 99% performance as raw IO.
>> >>
>> >
>> > This is very interesting...and impressive number.
>> >
>> > I tried to change ext4 to call blockdev_direct_IO_no_locking() directly,
>> > but then realize that we can't do this all the time, as ext4 support
>> > ext3 non-extent based files, and uninitialized extent is not support on
>> > ext3 format file.
>> >
>> >> As we understand, the reason why we want to take i_mutex lock during DIO
>> >> read is to prevent it from accessing stale data that may be exposed by a
>> >> simultaneous write. We saw that Mingming Cao has implemented a patch set
>> >> with which when a get_block request comes from direct write, ext4 only
>> >> allocates or splits an uninitialized extent. That uninitialized extent
>> >> will be marked as initialized at the end_io callback.
>> >
>> > Though I need to clarify that with all the patches in mainline, we only
>> > treat new allocated blocks form direct io write to holes, not to writes
>> > to the end of file. I actually have proposed to treat the write to the
>> > end of file also as unintialized extents, but there is some concerns
>> > that this getting tricky with updating inode size when it is async IO
>> > direct IO. So it didn't getting done yet.
>> >
>> >>  We are wondering
>> >> whether we can extend this idea to buffer write as well. I.e., we always
>> >> allocate an uninitialized extent first during any write and convert it
>> >> as initialized at the time of end_io callback. This will eliminate the need
>> >> to hold i_mutex lock during direct read because a DIO read should never get
>> >> a block marked initialized before the block has been written with new data.
>> >>
>> >
>> > Oh I don't think so. For buffered IO, the data is being copied to
>> > buffer, direct IO read would first flush what's in page cache to disk,
>>
>> Hmm, do you mean the filemap_write_and_wait_range() in
>> __blockdev_direct_IO?
>
> yes, that's the one to flush the page cache before direct read.
>
I don't think that function is called with DIO_NO_LOCKING.
Also, if we no longer hold i_mutex lock during dio read, I think
there is a time window that a buffer write can allocate an
initialize block after dio read flushes page cache but
before it calls get_block. Then that dio read can get that
initialized block with stale data.

Jiaying

>> Or do we flush page cache after calling
>> get_block in dio read?
>>
>> Jiaying
>>
>> > then read from disk. So if there is concurrent buffered write and direct
>> > read, removing the i_mutex locks from the direct IO path should still
>> > gurantee the right order, without having to treat buffered allocation
>> > with uninitialized extent/end_io.
>> >
>> > The i_mutex lock, from my understanding, is there to protect direct IO
>> > write to hole and concurrent direct IO read, we should able to remove
>> > this lock for extent based ext4 file.
>> >
>>
>>
>> >> We haven't implemented anything yet because we want to ask here first to
>> >> see whether this proposal makes sense to you.
>> >>
>> >
>> > It does make sense to me.
>> >
>> > Mingming
>> >> Regards,
>> >>
>> >> Jiaying
>> >> --
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>> >
>> >
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>
>
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