Re: O_DIRECT question

[Aubrey <aubreylee@gmail.com>]

On 1/11/07, Andrew Morton <akpm@osdl.org> wrote:
> On Thu, 11 Jan 2007 13:50:53 +0800
> Aubrey <aubreylee@gmail.com> wrote:
>
> > Firstly I want to say I'm working on no-mmu arch and uClinux.
> > After much of file operations VFS cache eat up all of the memory.
> > At this time, if an application request memory which order > 3, the
> > kernel will report failure.
>
> nommu kernels should probably run reclaim for higher-order allocations as
> well.

Here is the limitation. rebalance doesn't occur if order > 3.
/*
        * Don't let big-order allocations loop unless the caller explicitly
        * requests that.  Wait for some write requests to complete then retry.
        *
        * In this implementation, __GFP_REPEAT means __GFP_NOFAIL for order
        * <= 3, but that may not be true in other implementations.
        */
       do_retry = 0;
       if (!(gfp_mask & __GFP_NORETRY)) {
               if ((order <= 3) || (gfp_mask & __GFP_REPEAT))
                       do_retry = 1;
               if (gfp_mask & __GFP_NOFAIL)
                       do_retry = 1;
       }
       if (do_retry) {
               blk_congestion_wait(WRITE, HZ/50);
               goto rebalance;
       }

> That's rather a blunt instrument.  The "lumpy reclaim" patches in -mm
> provide a much better approach, but they need more work yet (although I
> don't immediately recall what's needed).

Thanks, I'll take a look.

> In the interim you could do the old "echo 3 > /proc/sys/vm/drop_caches"
> thing, but that's terribly crude - drop_caches is really only for debugging
> and benchmarking.
Yes. This method can drop caches, but will fragment memory. This is
not what I want. I want cache is limited to a tunable value of the
whole memory. For example, if total memory is 128M, is there a way to
trigger reclaim when cache size > 16M?

-Aubrey
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