Re: extremely long blockages when doing random writes to SSD

[Luigi Semenzato <semenzato@google.com>]

We're using CFQ.

CONFIG_DEFAULT_IOSCHED="cfq"
...
CONFIG_IOSCHED_CFQ=y
CONFIG_IOSCHED_DEADLINE=y
CONFIG_IOSCHED_NOOP=y

On Thu, Jun 25, 2015 at 5:58 PM, Sergey Senozhatsky
<sergey.senozhatsky.work@gmail.com> wrote:
> 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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