Messages in this thread | | | Date | Thu, 11 Jan 2007 14:45:12 +0800 | From | Aubrey <> | Subject | Re: O_DIRECT question |
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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?
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