Re: cp file /dev/zero <-> cache [was Re: increasing page size]

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Sun, 5 Jul 1998 19:00:04 +0200 (CEST), Rik van Riel
<H.H.vanRiel@phys.uu.nl> said:

> On Sun, 5 Jul 1998, Andrea Arcangeli wrote:
>> Where does the cache is allocated? Is it allocated in the inode? If so
>> kswapd should shrink the inode before start swapping out! 

> The cache is also mapped into a process'es address space.
> Currently we would have to walk all pagetables to find a
> specific page ;(

Not in this case, where the file is just being copied.  For a copy, the
reads exist unmapped in the page cache; only mmap() creates mapped
pages.


> When Stephen and Ben have merged their PTE stuff, we can
> do the freeing much easier though...

In this case, it's not an issue, so we need to fix it for 2.2.

>> I had to ask "2.0.34 has balancing code implemented and
>> running?". The

> 2.0 has no balancing code at all. At least, not AFAIK...

It does: the Duff's device in try_to_free_page does it, and seems to
work well enough.  It was certainly tuned tightly enough: all of the
hard part of getting the kswap stuff working well in try_to_swap_out()
was to do with tuning the aggressiveness of swap relative to the buffer
and cache reclaim mechanisms so that the try_to_free_page loop works
well.  That's why the recent policies of adding little rules here and
there all over the mm layer have disturbed the balance so much, I think.

>> Is there a function call (such us shrink_mmap for mmap or
>> kmem_cache_reap() for slab or shrink_dcache_memory() for dcache) that
>> is able to shrink the cache allocated by cp file /dev/zero?

> shrink_mmap() can only shrink unlocked and clean buffer pages
> and unmapped cache pages. We need to go through either bdflush
> (for buffer) or try_to_swap_out() first, in order to make some
> easy victims for shrink_mmap()...

Only for mapped files, not files copied through the standard read/write
calls.

--Stephen

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