On 07/10/2015 13:49, hiren panchasara wrote:
> On 07/09/15 at 06:13P, Alan Cox wrote:
>> On Jul 8, 2015, at 1:29 PM, hiren panchasara <hi...@freebsd.org> wrote:
>>
>>> On 07/08/15 at 05:46P, Alan Cox wrote:
>>>> Author: alc
>>>> Date: Wed Jul 8 17:45:59 2015
>>>> New Revision: 285282
>>>> URL: https://svnweb.freebsd.org/changeset/base/285282
>>>>
>>>> Log:
>>>> The intention of r254304 was to scan the active queue continuously.
>>>> However, I've observed the active queue scan stopping when there are
>>>> frequent free page shortages and the inactive queue is steadily refilled
>>>> by other mechanisms, such as the sequential access heuristic in vm_fault()
>>>> or madvise(2).
>>> What would be the actual downside/effect of this scenario? i.e. What goes
>>> wrong when active queue scan stops and what would I see/observe on a
>>> system when the problem is going on - is what I want to understand.
>>>
>>
>> Basically, pages that haven?t been used recently get stuck in the active
>> queue. Consequently, they are never reclaimed and repurposed to hold newly
>> accessed data. Instead, more recently used pages from the inactive queue
>> are reclaimed. Effectively, this reduces the amount of physical memory that
>> is available for caching file data, and so the system (likely) performs more
>> I/O operations than it would have with the stuck pages problem corrected.
>>
> Thanks. So it seems now there is an enforced number of min pages that
> *must* be scanned which is proportional to the time of last scan.
That is correct.
>>>> To remedy this problem, record the time of the last active
>>>> queue scan, and always scan a number of pages proportional to the time
>>>> since the last scan, regardless of whether that last scan was a
>>>> timeout-triggered ("pass == 0") or free-page-shortage-triggered ("pass >
>>>> 0") scan.
>>>>
>>>> Also, on a timeout-triggered scan, allow a full scan of the active queue
>>>> when the system is short of inactive pages.
>>>>
>>>> Reviewed by: kib
>>>> MFC after: 6 weeks
>>>> Sponsored by: EMC / Isilon Storage Division
>>> [skip]
>>>
>>> Thanks in advance.
>>> Cheers,
>>> Hiren
>>
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