Re: [PATCH 3/3] readahead: scale max readahead size depending on memory size

[Jens Axboe <jens.axboe@oracle.com>]

On Sun, Jul 22 2007, Rik van Riel wrote:
> Jens Axboe wrote:
>
>> I just wish you had a rationale behind them, I don't think it's that
>> great of a series. I agree with the low point of 128k. Then it'd be sane
>> to try and determine what the upper limit of ra window size goodness is,
>> which is probably impossible since it depends on the hardware a lot. But
>> lets just say the upper value is 2mb, then I think it's pretty silly
>> _not_ to use 2mb on a 1g machine for instance. So more aggressive
>> scaling.
>
> 1 or 2 MB is a nice number.
>
> Seek time (plus rotational latency) on disks still takes
> on the order of 10 ms, while disks commonly transfer data
> on the order of 50MB/second.
>
> That means one disk seek (10ms) takes as long as it takes
> to read around 512kB of data.
>
> The current 128kB means that if you have lots of streaming
> IO going on, you spend only 20% of the time transferring
> data and get roughly 10MB/s.  Seek 10ms, read 2.5ms worth
> of data.
>
> OTOH, if you do 2MB per request for the same heavy streaming
> workload (say, an ftp or nfs server doing media files), you
> can get 80% of the disk throughput, or 40MB/s.  This is because
> you spend 40ms transferring data for every 10ms seek time.
>
> Yes, filesystem metadata will reduce this "occasionally",
> but the general idea holds.

I meant real numbers, the above is just rudimentary math. AS and CFQ
makes sure that you get a window of more than one request in, so huge
read-ahead sizes aren't as interesting.

So - more testing, less hand waving :-)

-- 
Jens Axboe

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