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

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Date:	Tue, 31 Mar 2009 15:24:03 +0300
From:	Izik Eidus <ieidus@...hat.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
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.

Anthony Liguori wrote:
> 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?

I belive using ioctl for registering memory of applications make it 
easier....
Ksm doesnt have any complicated API that would benefit from sysfs 
(beside adding more complexity)

> That is, the KSM_START_STOP_KTHREAD part, not necessarily the rest of 
> the API.

What you mean?
>
> Regards,
>
> Anthony Liguori
>

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