Re: extremely long blockages when doing random writes to SSD

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

Hello,

On (06/25/15 11:24), Luigi Semenzato wrote:
> I looked at this some more and I am not sure that there is any bug, or
> other possible tuning.
> 
> While the random-write process runs, iostat -x -k 1 reports these numbers:
> 
> average queue size: around 300
> average write wait: typically 200 to 400 ms, but can be over 1000 ms
> average read wait: typically 50 to 100 ms
> 
> (more info at crbug.com/414709)
> 
> The read latency may be enough to explain the jank.  In addition, the
> browser can do fsyncs, and I think that those will block for a long
> time.
> 
> Ionice doesn't seem to make a difference.  I suspect that once the
> blocks are in the output queue, it's first-come/first-serve.  Is this
> correct or am I confused?
> 
> We can fix this on the application side but only partially.  The OS
> version updater can use O_SYNC.  The problem is that his can happen in
> a number of situations, such as when simply downloading a large file,
> and in other code we don't control.
> 

do you use CONFIG_IOSCHED_DEADLINE or CONFIG_IOSCHED_CFQ?

	-ss

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