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From Robert Newson <rnew...@apache.org>
Subject Re: few doubts
Date Mon, 04 Jul 2011 21:13:13 GMT
>From (vague) memory, I thought page cache was released when you close
the file descriptor.

B.

On 4 July 2011 21:55, Randall Leeds <randall.leeds@gmail.com> wrote:
> On Mon, Jul 4, 2011 at 13:34, Randall Leeds <randall.leeds@gmail.com> wrote:
>>
>> On Jul 4, 2011 11:11 AM, "Jens Alfke" <jens@mooseyard.com> wrote:
>>>
>>>
>>> On Jul 3, 2011, at 11:41 PM, Matthew John wrote:
>>>
>>> > how does CouchDB perform
>>> > when compared to MySql in performing the Warm-up (lets define Warm-up as
>>> > filling the memory with Disk data in a judicial manner).
>>>
>>> No idea. But really, you could perform something very similar by doing
>>> something like
>>>        cat /path/to/my_database.couch > /dev/null
>>> since that would have the same effect of loading the file contents into
>>> the VM cache. So it’s not really that specific to CouchDB itself.
>>>
>>> How large is your data-set? How many rows, and what’s the disk space
>>> occupied by the MySQL tables?
>>>
>>> —Jens
>>
>> It should also be noted that CouchDB does not do any caching of its own. I
>> don't know if MySQL does, but if yes then your warm-up query is possibly
>> letting MySQL cache some index headers or something, but nothing exactly
>> like this happens in CouchDB. The only caching is at the filesystem/OS
>> level. In fact, if your OS hasn't decided to dump the page cache, it may be
>> beneficial _not_ to do anything that might disturb it between restarts,
>> since some of that file data might still be around. This behavior would
>> depend on your OS keeping the pages around after all file descriptors are
>> closed. I don't know whether that happens or when. Maybe there is a kernel
>> setting for this behavior?
>
> Ah. Of course. The kernel parameter is /sys/proc/vm/swappiness on Linux systems.
> 1 - Dump cached pages in favor of keeping applications in memory
> ...
> ...
> 100 - Swap applications in favor of keeping pages cached in memory
>
> I suspect there's no aggressive dumping of pages just because files
> have been closed, though I haven't found any information one way or
> another yet. In other words, while your warm-up query might do things
> for MySQL's internal caching that could have benefits, doing something
> similar in CouchDB would probably just force out whatever useful pages
> your OS has cached from before the restart. I would not recommend
> trying to do anything like this.
>

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