Re: IO scheduler benchmarking

[Andrew Morton <akpm@digeo.com>]

Andrea Arcangeli <andrea@suse.de> wrote:
>
> I don't
> buy Andrew complaining about the write throttling when he still allows
> several dozen mbytes of ram in flight and invisible to the VM,

The 2.5 VM accounts for these pages (/proc/meminfo:Writeback) and throttling
decisions are made upon the sum of dirty+writeback pages.

The 2.5 VFS limits the amount of dirty+writeback memory, not just the amount
of dirty memory.

Throttling in both write() and the page allocator is fully decoupled from the
queue size.  An 8192-slot (4 gigabyte) queue on a 32M machine has been
tested.

The only tasks which block in get_request_wait() are the ones which we want
to block there: heavy writers.

Page reclaim will never block page allocators in get_request_wait().  That
causes terrible latency if the writer is still active.

Page reclaim will never block a page-allocating process on I/O against a
particular disk block.  Allocators are instead throttled against _any_ write
I/O completion.  (This is broken in several ways, but it works well enough to
leave it alone I think).