Re: [rfc] [patch] mm: zone_reclaim fix for pseudo file systems

Christoph Lameter
2007-07-31T05:33:03Z

On Mon, 30 Jul 2007, Andrew Morton wrote:

> Nonsense. The VM used to handle it just fine. That's what I wrote the
> all_unreclaimable logic *for*. It wasn't just added as typing practice.

That is if the whole zone is unreclaimable. The problems that we want to
solve are due to parts of a zone being unreclaimable and due to the VM
counters giving an inaccurate picture of the memory situation.

> Here's the changelog, from 22 Nov 2002:
>
> [PATCH] handle zones which are full of unreclaimable pages
>
> This patch is a general solution to the situation where a zone is full
> of pinned pages.
>
> This can come about if:
>
> a) Someone has allocated all of ZONE_DMA for IO buffers
>
> b) Some application is mlocking some memory and a zone ends up full
> of mlocked pages (can happen on a 1G ia32 system)
>
> c) All of ZONE_HIGHMEM is pinned in hugetlb pages (can happen on 1G
> machines)
>
> We'll currently burn 10% of CPU in kswapd when this happens, although
> it is quite hard to trigger.
>
> The algorithm is:
>
> - If page reclaim has scanned 2 * the total number of pages in the
> zone and there have been no pages freed in that zone then mark the
> zone as "all unreclaimable".
>
> - When a zone is "all unreclaimable" page reclaim almost ignores it.
> We will perform a "light" scan at DEF_PRIORITY (typically 1/4096'th of
> the zone, or 64 pages) and then forget about the zone.
>
> - When a batch of pages are freed into the zone, clear its "all
> unreclaimable" state and start full scanning again. The assumption
> being that some state change has come about which will make reclaim
> successful again.
>
> So if a "light scan" actually frees some pages, the zone will revert to
> normal state immediately.
>
> So we're effectively putting the zone into "low power" mode, and lightly
> polling it to see if something has changed.
>
> The code works OK, but is quite hard to test - I mainly tested it by
> pinning all highmem in hugetlb pages.
>
>
> See? "general".

Nope. Its a special situation in which the whole zone has become
unhandleable by the reclaim logic so it gives up and waits for things
somehow to get better. During that time we cannot allocate from a zone
which typically makes a vital zone or a node unusuable. In a NUMA
configuration performance degrades in unacceptable ways.

What we want is to remove the unreclaimable pages from the LRU and have
reclaim continue on the remainder of the zone.

> No, let us not. If the existing crap isn't working as it should (and as it
> used to) let us first fix (or at least understand) that before adding more
> crap.
>
> No?

The all_reclaimable logic is different. It was never been designed to
remove the unreclaimable pages.