lkml.org 
[lkml]   [2007]   [Jan]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Abysmal disk performance, how to debug?
On Sat, 20 Jan 2007, Willy Tarreau wrote:
> On Sat, Jan 20, 2007 at 09:39:25PM +0100, Tim Schmielau wrote:
> > On Sat, 20 Jan 2007, Willy Tarreau wrote:
> > > On Sat, Jan 20, 2007 at 09:10:22PM +0100, Tim Schmielau wrote:
> > >
> > > > also explains why you see better results is writeout starts earlier.
> > >
> > > The results should be better when writeout starts later since most of
> > > the transfer will have been performed at RAM speed. That's what happens
> > > with the user above with 2 GB RAM. But in case of the VAIO with 512 MB,
> > > there's really something strange IMHO. I suspect it has two RAM areas,
> > > one fast and one slow (probably one two large non-cacheable area for a
> > > shared video or such a crap, which can be avoided when reducing the
> > > cache size).
> >
> > No - the earlier the writeout starts, the earlier he will have enough free
> > RAM to finish the dd command by buffering the remaining data.
>
> OK I see your point. While trying to show why I got you wrong, I in fact
> demonstrated to myself that you were right :-)
>
> For instance, let's say we have 500 MB cache at 1000 MB/s and a write out
> threshold of 80% with a disk at 100 MB/s. Writing 1000 MB would produce
> this pattern :
>
> time data sent written dirty data
> in sec from dd to disk in cache
> 0.0 0 MB 0 MB 0 MB
> 0.4 400 MB 0 MB 400 MB -> writeout starts
> 1.0 560 MB 60 MB 500 MB
> 5.4 1000 MB 500 MB 500 MB -> dd leaves.
> 10.4 1000 MB 1000 MB 0 MB -> write terminated.
>
> -> avg dd speed = 1000/5.4 = 185 MB/s
> avg disk speed = 1000/10.4 = 96 MB/s
>
>
> Now with a lower writeout threshold of 2% (10 MB) :
>
> time data sent written dirty data
> in sec from dd to disk in cache
> 0.0 0 MB 0 MB 0 MB
> 0.01 10 MB 0 MB 10 MB -> writeout starts
> 1.0 599 MB 99 MB 500 MB
> 5.01 1000 MB 500 MB 500 MB -> dd leaves.
> 10.01 1000 MB 1000 MB 0 MB -> write terminated.
>
> -> avg dd speed = 1000/5.01 = 199.6 MB/s
> avg disk speed = 1000/10.01 = 99.9 MB/s
>
> At least, numbers are not that much different to justify a one to two speed
> ratio on the VAIO. The difference being caused by cache speed, it's clearly
> possible that his RAM is definitely very very slow which would then explain
> the difference.
>
> ----
> > Note that we did not cap the amount of buffers, just started to write out
> > earlier.
> ----
>
> Indeed, that's what makes the whole difference. I was used to cap the amount
> of buffers, but the behaviour here is different.
>
> Thanks for your insight !

Thanks for being so humble in pointing out my logic is flawed. While the
Vaio certainly cannot write 1000GB/s to its RAM, it's disk is also quite
slow and the ratio of 10:1 for RAM:disk speed is presumably correct.
So we don't quite understand why dd in RAM is so slow for him.

Thanks,
Tim
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-01-20 23:29    [W:0.094 / U:0.076 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site