Re: why my systems never cache more than ~900 MB?

[Tomasz Chmielewski <mangoo@wpkg.org>]

Nick Piggin schrieb:
> On Tuesday 24 March 2009 19:42:08 Tomasz Chmielewski wrote:
>> On my (32 bit) systems with more than 1 GB memory it is impossible to cache
>> more than about 900 MB. Why?
>>
>> Caching never goes beyond ~900 MB (i.e. when I read a mounted drive with
>> dd):
> 
> Because blockdev mappings are limited to lowmem due to sharing their
> cache with filesystem metadata cache, which needs kernel mapped memory.
> It will >900MB of pagecache data OK (data from regular files)

Does not help me, as what interests me here on these machines is mainly 
caching block device data; they are iSCSI targets and access block 
devices directly.

(...)

>> Same behaviour on 32 bit machines with 4 GB RAM.
>>
>> No problems on 64 bit machines.
>> I have one 32 bit machine that caches beyond ~900 MB without problems.
> 
> Does it have a different user/kernel split?

Yes it does:

CONFIG_VMSPLIT_2G_OPT=y


What split should I choose to enable blockdev mapping on the whole 
memory on 32 bit system with 3 or 4 GB RAM? Is it possible with 4 GB RAM 
at all?


>> Is it some kernel/proc/sys setting that I'm missing?
> 
> No, it just can't be done without changing code.


-- 
Tomasz Chmielewski
http://wpkg.org

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