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Re: HFS Patch 15 out

From: Szakacsits Szabolcs
Subject: Re: HFS Patch 15 out
Date: Mon, 27 Sep 2004 13:17:20 +0200 (MEST)

On Fri, 24 Sep 2004, K.G. wrote:

> I've rewritten the search algorithm using a cache storing extent
> references of the FS in a sorted order, but this has not resulted in
> the spectacular speed improvement I expected. I've found that most
> (indeed nearly all) CPU time is spent in kernel space so I will try to
> understand why 

On most of todays commodity hardwares the disk IO (speed and seek) should
be the bottleneck. For example, if I see it right, you use a lot sync's.
That's definitely a big performance killer, the kernel can't merge,
optimize disk IO and often the resize process just blocks waiting for IO.

Here are two ntfsresize timings (for relocating 3 GB) using in sync
intensive (data should be consistent even in case of power outage) and
less sync intensive mode (data might be corrupt in case of power outage):

   8.43s  usr,  46.85s sys,  179.16s real,  30% CPU  <-- careful sync's
  39.16s  usr,  59.76s sys,  843.82s real,  11% CPU  <--  lot of sync's

In this case, careful sync gave almost 5x speedup.

Ntfsresize's block allocator also tries to minimize disk seeks thus the
kernel can merge the IO write request.

> and find new solutions to accelerate things.

Doing it efficiently _and_ safely from user space can be somehow
complicated because the kernel doesn't provide the needed support, 
nor it exports needed functionality (e.g. setting write barriers).


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