Re: madvise failure

[Andries E. Brouwer]

On Fri, May 08, 2009 at 10:07:45AM +0900, KAMEZAWA Hiroyuki wrote:
> "Andries E. Brouwer" <Andries.Brouwer@xxxxxx> wrote:
> 
>> In an application something like
>> 	p = mmap(0, sz, PROT_READ, MAP_PRIVATE, fd, 0);
>> 	madvise(p, sz, MADV_SEQUENTIAL);
>> is done for a small number of files, each with a size of a few GB.
>> A single sequential pass is done over these files - essentially a merge.
>> 
>> On an old machine the madvise is useful, and decreases total time.

> With the same kernel (Ubuntu/2.6.27) ? old one ?

Both Ubuntu/2.6.27: 2.6.27-11-generic (new) vs 2.6.27-11-server (old)

>> But on a more recent machine, with more memory, the madvise makes
>> things worse. There, it seems better not to reveal to the kernel
>> that the data will be read sequentially.
>> 
>> Timing (six files of 4GB each, quadcore Intel Q9550, 16GB memory,
>> kernel 2.6.27 [Ubuntu], two other processes active):
>> with madvise, 7 runs: real time varying 9m10s - 37m29s,
>> without madvise, 6 runs: real time fairly constant 5m45s - 5m54s.

> Accessing each page one by one sequentially ?

Yes. Essentially a merge.

> or Sequential but (may) skip some pages in each access ?

No.

Andries

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