[svlug] SSD was: Slides from the GoLUG meeting

Ivan Sergio Borgonovo mail at webthatworks.it
Fri Jun 3 03:18:34 PDT 2016

On 06/03/2016 10:34 AM, Rick Moen wrote:
> Quoting Ivan Sergio Borgonovo (mail at webthatworks.it):
>> oh it does.
>> https://www.kernel.org/doc/Documentation/block/cfq-iosched.txt
>> "CFQ has some optimizations for SSDs and if it detects a non-rotational
>> media which can support higher queue depth (multiple requests at in
>> flight at a time), then it cuts down on idling of individual queues and
>> all the queues move to sync-noidle tree and only tree idle remains. This
>> tree idling provides isolation with buffered write queues on async tree."

> The point is, people who've studied this matter carefully say the Noop
> and Deadline schedulers are simply better for such media, less impairing
> of I/O, less wasteful of CPU cycles doing inapplicable things.  I see no
> reason to doubt them, or to think that CFQ with some functions disabled
> is better still.

> What I cannot understand is why you think the quoted text shows that the
> kernel can do a better job than you.  It doesn't show that at all, and
> also good reasons why it's a doubtful assertion stand on their merits
> irrespective of what that file says.

I think I've read it from "people who've studied this matter carefully".
Sorry I couldn't track down where I read it, but the memories I have 
about it, and they could be faulty were "you shouldn't care anymore".

I think deciding what's best is *now* more complicated than "scheduler A 
is simpler and it looks more adequate".
I didn't find many recent benchmark comparing cfq to deadline since 
there is a new scheduler that is attracting attentions (bfq) but reading


it looks as it is at least debatable which scheduler to chose.

I'm just suggesting that if anyone really really need to know should do 
his homework again.

Ivan Sergio Borgonovo

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