linux-kernel - Re: [PATCH 4/4] add ksm kernel shared memory driver.

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Date:	Mon, 30 Mar 2009 21:12:20 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Izik Eidus <ieidus@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-mm@...ck.org, avi@...hat.com, aarcange@...hat.com,
	chrisw@...hat.com, riel@...hat.com, jeremy@...p.org,
	mtosatti@...hat.com, hugh@...itas.com, corbet@....net,
	yaniv@...hat.com, dmonakhov@...nvz.org
Subject: Re: [PATCH 4/4] add ksm kernel shared memory driver.

Izik Eidus wrote:
> Ksm is driver that allow merging identical pages between one or more
> applications in way unvisible to the application that use it.
> Pages that are merged are marked as readonly and are COWed when any
> application try to change them.
>
> Ksm is used for cases where using fork() is not suitable,
> one of this cases is where the pages of the application keep changing
> dynamicly and the application cannot know in advance what pages are
> going to be identical.
>
> Ksm works by walking over the memory pages of the applications it
> scan in order to find identical pages.
> It uses a two sorted data strctures called stable and unstable trees
> to find in effective way the identical pages.
>
> When ksm finds two identical pages, it marks them as readonly and merges
> them into single one page,
> after the pages are marked as readonly and merged into one page, linux
> will treat this pages as normal copy_on_write pages and will fork them
> when write access will happen to them.
>
> Ksm scan just memory areas that were registred to be scanned by it.
>
> Ksm api:
>
> KSM_GET_API_VERSION:
> Give the userspace the api version of the module.
>
> KSM_CREATE_SHARED_MEMORY_AREA:
> Create shared memory reagion fd, that latter allow the user to register
> the memory region to scan by using:
> KSM_REGISTER_MEMORY_REGION and KSM_REMOVE_MEMORY_REGION
>
> KSM_START_STOP_KTHREAD:
> Return information about the kernel thread, the inforamtion is returned
> using the ksm_kthread_info structure:
> ksm_kthread_info:
> __u32 sleep:
>         number of microsecoends to sleep between each iteration of
> scanning.
>
> __u32 pages_to_scan:
>         number of pages to scan for each iteration of scanning.
>
> __u32 max_pages_to_merge:
>         maximum number of pages to merge in each iteration of scanning
>         (so even if there are still more pages to scan, we stop this
> iteration)
>
> __u32 flags:
>        flags to control ksmd (right now just ksm_control_flags_run
> 			      available)
>   

Wouldn't this make more sense as a sysfs interface?  That is, the 
KSM_START_STOP_KTHREAD part, not necessarily the rest of the API.

Regards,

Anthony Liguori

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