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Date: Wed, 28 Jan 2015 15:36:02 -0800
From: Nikhilesh Reddy <reddyn@...eaurora.org>
To: "Darrick J. Wong" <darrick.wong@...cle.com>
CC: linux-ext4@...r.kernel.org
Subject: Re: Writes blocked on wait_for_stable_page (Writes of less than page
size sometimes take too long)
Also can someone please confirm if the following patch help address
this issue That I described below?
https://kernel.googlesource.com/pub/scm/linux/kernel/git/tytso/ext4/+/7afe5aa59ed3da7b6161617e7f157c7c680dc41e
Quick Summary:
in the function
wait_on_page_writeback(page) inside ext4_da_write_begin seems to take
long and the process is stuck waiting.
The writeback just seems to take long.. but only when we write to the
same page over again...
The grab_cache_page_write_begin gives the same page until we write
enough...
Is there a wait to not wait for the writeback?
On 01/28/2015 03:23 PM, Nikhilesh Reddy wrote:
>
> Hi Darrick
> Thanks so much for your reply.
> I apologize the stable page wait seems to be an odd outlier in that
> run ... I ran multiple traces again.
>
> Please find my answers inline below.
>
> On 01/28/2015 01:39 PM, Darrick J. Wong wrote:
>> On Wed, Jan 28, 2015 at 11:27:13AM -0800, Nikhilesh Reddy wrote:
>>> Hi
>>> I am working on a 64 bit Android device and have been trying to
>>> improve performance for stream based data download (for example an
>>> ftp)
>>> The device has 3GB of ram and the dirty_ratio and
>>> dirty_background_ratio are set to 5 and 1 respectively.
>>>
>>> Kernel 3.10 , Highmem is not enabled and the backing device is a
>>> emmc and checksumming is not enabled
>> Ok, 3.10 kernel is new enough that stable page writes only apply to
>> devices that demand it, and apparently your eMMC demands it.
> I tried checking my logs again .. and yes the stable pages dont apply
> ... that seems to be an odd instance where it took a while.
> /My apologies.
>
> So On running many more trace runs it appears that the wait is actually
> on in the function
>
> wait_on_page_writeback(page) ext4_da_write_begin
>
> Snippet below
>
> /*
> * grab_cache_page_write_begin() can take a long time if the
> * system is thrashing due to memory pressure, or if the page
> * is being written back. So grab it first before we start
> * the transaction handle. This also allows us to allocate
> * the page (if needed) without using GFP_NOFS.
> */
> retry_grab:
> page = grab_cache_page_write_begin(mapping, index, flags);
> if (!page)
> return -ENOMEM;
> unlock_page(page);
>
> retry_journal:
> handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE, needed_blocks);
> if (IS_ERR(handle)) {
> page_cache_release(page);
> return PTR_ERR(handle);
> }
>
> lock_page(page);
> if (page->mapping != mapping) {
> /* The page got truncated from under us */
> unlock_page(page);
> page_cache_release(page);
> ext4_journal_stop(handle);
> goto retry_grab;
> }
> wait_on_page_writeback(page); <<<<<<<<<<<<<<<<<<This is where the
> wait is!
>
>
>
> The writeback just seems to take long.. but only when we write to the
> same page over again...
> The grab_cache_page_write_begin gives the same page until we write
> enough...
>
> Is there a wait to not wait for the writeback?
>
>
>>
>>> I noticed when profiling writes that if we dont use streamed IO (ie.
>>> use write of whatever size data was read on the tcp stream) there
>>> are some writes that seem to get blocked on
>>> wait_for_stable_page.
>>>
>>> If I force the writes to be buffered in the userspace and ensure
>>> writing 4k chunks the writes never seem to stall.
>> That's consistent with a page being partially dirtied, written out,
>> and partially dirtied again before write-out finishes. If you buffer
>> the incoming data such that a page is only dirtied once, you'll never
>> notice wait_for_stable_page.
>>
>> Are you explicitly forcing writeout (i.e. fsync()) after every chunk
>> arrives? Or, is the rate of incoming data high enough such that we
>> hit either dirty*ratio limit? It isn't too hard to hit 30MB these
>> days. Why are you lowering the ratios from their defaults?
> Using the higher values seems to cause longer stall ...when we do stall...
> Smaller values are cause the write out to happen often but each one
> takes less time.
>
> Setting it to the defaults causes a longer stall which seems to be
> impacting TCP due to the delay in reading.
> Additional info : dirty_expire_centisecs was set to 200 and the data
> rate is close to 300Mbps. so yea 30MB should be less than a second.
>
> Am I missing something obvious here?
>
> Please do let me know.
>
> Thanks so much for you help
>
>
>>
>>> I noticed there was earlier discussion on this and idea were
>>> proposed to use snapshotting of the pages to avoid stalls...
>>> For example: https://lwn.net/Articles/546658/
>>>
>>> But this seems to only snapshot ext3 ... (unless i misunderstood
>>> what the patch is doing)
>>>
>>> Is there a similar patch to snapshot the buffers to not stall the
>>> writes for ext4?
>> No, there is not -- the problem with the snapshot solution is that it
>> requires page allocations when the FS is (potentially) trying to
>> reclaim memory by writing out dirty pages.
>>
>> --D
>>
>>> Please let me know.
>>>
>>> I would really appreciate any help you can give me.
>>>
>>>
>>> --
>>> Thanks
>>> Nikhilesh Reddy
>>>
>>> Qualcomm Innovation Center, Inc.
>>> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
>>> Forum,
>>> a Linux Foundation Collaborative Project.
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
>>> the body of a message to majordomo@...r.kernel.org
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>
>
--
Thanks
Nikhilesh Reddy
Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.
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