Re: extremely long blockages when doing random writes to SSD

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 24 Jun 2015 14:54:09 -0700 Luigi Semenzato <semenzato@google.com> wrote:

> Greetings,
> 
> we have an app that writes 4k blocks to an SSD partition with more or
> less random seeks.  (For the curious: it's called "update engine" and
> it's used to install a new Chrome OS version in the background.)  The
> total size of the writes can be a few hundred megabytes.  During this
> time, we see that other apps, such as the browser, block for seconds,
> or tens of seconds.
> 
> I have reproduced this behavior with a small program that writes 2GB
> worth of 4k blocks randomly to the SSD partition.  I can get apps to
> block for over 2 minutes, at which point our hang detector triggers
> and panics the kernel.
> 
> CPU: Intel Haswell i7
> RAM: 4GB
> SSD: 16GB SanDisk
> kernel: 3.8
> 
> >From /proc/meminfo I see that the "Buffers:" entry easily gets over
> 1GB.  The problem goes away completely, as expected, if I use O_SYNC
> when doing the random writes, but then the average size of the I/O
> requests goes down a lot, also as expected.
> 
> First of all, it seems that there may be some kind of resource
> management bug.  Maybe it has been fixed in later kernels?  But, if
> not, is there any way of encouraging some in-between behavior?  That
> is, limit the allocation of I/O buffers to a smaller amount, which
> still give the system a chance to do some coalescing, but perhaps
> avoid the extreme badness that we are seeing?
> 

What kernel version?

Are you able to share that little test app with us?

Which filesystem is being used and with what mount options etc?



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