Re: ext4 DIO read performance issue on SSD

Jiaying Zhang
2009-10-15T23:34:31Z

On Thu, Oct 15, 2009 at 4:28 PM, Mingming <cmm@us.ibm.com> wrote:
> On Thu, 2009-10-15 at 13:07 -0700, Jiaying Zhang wrote:
>> On Thu, Oct 15, 2009 at 10:31 AM, Mingming <cmm@us.ibm.com> wrote:
>> > On Wed, 2009-10-14 at 22:14 -0700, Jiaying Zhang wrote:
>> >> Mingming,
>> >>
>> >
>> > Hi Jiaying,
>> >
>> >> 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.
>> >>
>> >> I read you previous email thread again. As I understand, the main
>> >> concern for allocating uninitialized blocks in i_size extending write
>> >> is that we may end up having uninitialized blocks beyond i_size
>> >> if the system crashes during write. Can we protect this case by
>> >> adding the inode into the orphan list in ext4_ext_direct_IO,
>> >> i.e., same as we have done in ext4_ind_direct_IO?
>> >>
>> >
>> > Sure we could do that, though initially I hoped we could get rid of
>> > that:)
>> >
>> > The tricky part is async direct write to the end of file. By the time
>> > the IO is completed, the inode may be truncated or extended larger.
>> > Updating the most "safe" size is the part I haven't thought through yet.
>> >
>>
>> Ok. I think I understand the problem better now :).
>>
>> Looking at the __blockdev_direct_IO(), I saw it actually treats
>> size-extending aio dio write as synchronous and expects the dio to
>> complete before return (fs/direct-io.c line 1204 and line 1056-1061).
>
> Oh? It seems it will keep the write async as long as it's not a partial
> write
>        /*
>         * The only time we want to leave bios in flight is when a successful
>         * partial aio read or full aio write have been setup.  In that case
>         * bio completion will call aio_complete.  The only time it's safe to
>         * call aio_complete is when we return -EIOCBQUEUED, so we key on that.
>         * This had *better* be the only place that raises -EIOCBQUEUED.
>         */
>        BUG_ON(ret == -EIOCBQUEUED);
>        if (dio->is_async && ret == 0 && dio->result &&
>            ((rw & READ) || (dio->result == dio->size)))
>                ret = -EIOCBQUEUED;
>
>        if (ret != -EIOCBQUEUED)
>                dio_await_completion(dio);
>

If I read the code correctly, dio->is_async is not set for file extending write:

/*
* For file extending writes updating i_size before data
* writeouts complete can expose uninitialized blocks. So
* even for AIO, we need to wait for i/o to complete before
* returning in this case.
*/
dio->is_async = !is_sync_kiocb(iocb) && !((rw & WRITE) &&
(end > i_size_read(inode)));

Jiaying

>> Can we follow the same rule and check whether it is a size-extending
>> aio write in ext4_end_io_dio()? In such cases, we can call
>> ext4_end_aio_dio_nolock() synchronously instead of queuing
>> the work. I think this will protect us from truncate because we
>> are still holding i_mutex and i_alloc_sem.
>>
>> Jiaying
>>
>> >
>> >
>> >> Jiaying
>> >>
>> >> >
>> >> >>  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,
>> >> > 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
>> >> >> --
>> >> >> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
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>> >> >
>> >> >
>> >> >
>> >
>> >
>> >
>> --
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>
>
>
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