linux-kernel - Re: [PATCH] filesystem-dax: Disable PMD support

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Date:   Wed, 3 Jul 2019 03:22:49 +0300
From:   Boaz Harrosh <openosd@...il.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Seema Pandit <seema.pandit@...el.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Robert Barror <robert.barror@...el.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] filesystem-dax: Disable PMD support

On 02/07/2019 18:37, Dan Williams wrote:
<>
> 
> I'd be inclined to do the brute force fix of not trying to get fancy
> with separate PTE/PMD waitqueues and then follow on with a more clever
> performance enhancement later. Thoughts about that?
> 

Sir Dan

I do not understand how separate waitqueues are any performance enhancement?
The all point of the waitqueues is that there is enough of them and the hash
function does a good radomization spread to effectively grab a single locker
per waitqueue unless the system is very contended and waitqueues are shared.
Which is good because it means you effectively need a back pressure to the app.
(Because pmem IO is mostly CPU bound with no long term sleeps I do not think
 you will ever get to that situation)

So the way I understand it having twice as many waitqueues serving two types
will be better performance over all then, segregating the types each with half
the number of queues.

(Regardless of the above problem of where the segregation is not race clean)

Thanks
Boaz