Re: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone pressure

[Rik van Riel <riel@redhat.com>]

On 04/21/2010 03:35 AM, Christian Ehrhardt wrote:
>
>
> Christian Ehrhardt wrote:
>>
>>
>> Rik van Riel wrote:
>>> On 04/20/2010 11:32 AM, Johannes Weiner wrote:
>>>
>>>> The idea is that it pans out on its own. If the workload changes, new
>>>> pages get activated and when that set grows too large, we start
>>>> shrinking
>>>> it again.
>>>>
>>>> Of course, right now this unscanned set is way too large and we can end
>>>> up wasting up to 50% of usable page cache on false active pages.
>>>
>>> Thing is, changing workloads often change back.
>>>
>>> Specifically, think of a desktop system that is doing
>>> work for the user during the day and gets backed up
>>> at night.
>>>
>>> You do not want the backup to kick the working set
>>> out of memory, because when the user returns in the
>>> morning the desktop should come back quickly after
>>> the screensaver is unlocked.
>>
>> IMHO it is fine to prevent that nightly backup job from not being
>> finished when the user arrives at morning because we didn't give him
>> some more cache - and e.g. a 30 sec transition from/to both optimized
>> states is fine.
>> But eventually I guess the point is that both behaviors are reasonable
>> to achieve - depending on the users needs.
>>
>> What we could do is combine all our thoughts we had so far:
>> a) Rik could create an experimental patch that excludes the in flight
>> pages
>> b) Johannes could create one for his suggestion to "always scan active
>> file pages but only deactivate them when the ratio is off and
>> otherwise strip buffers of clean pages"

I think you are confusing "buffer heads" with "buffers".

You can strip buffer heads off pages, but that is not
your problem.

"buffers" in /proc/meminfo stands for cached metadata,
eg. the filesystem journal, inodes, directories, etc...
Caching such metadata is legitimate, because it reduces
the number of disk seeks down the line.

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