Re: [PATCH] makedumpfile: Use file offset in initialize_mmap()

[Petr Tesarik <ptesarik@suse.cz>]

On Wed, 4 Mar 2015 13:44:18 +0100
Michael Holzheu <holzheu@linux.vnet.ibm.com> wrote:

> On Tue, 3 Mar 2015 11:07:50 +0100
> Petr Tesarik <ptesarik@suse.cz> wrote:
> 
> > On Tue, 3 Mar 2015 10:15:43 +0100
> > Michael Holzheu <holzheu@linux.vnet.ibm.com> wrote:
> 
> [snip]
> 
> > > I did a quick test with your patch and it looks like the mmap mode
> > > on my s390 system is slower than the read mode:
> > 
> > That's sad. OTOH I had similar results on a file mmap some time ago.
> > The cost of copying data was less than the cost of handling a series of
> > minor page faults.
> 
> I think we understood the problem: As for the read path, also for mmap
> the memory is copied into a temporary buffer:
> 
>  static int read_with_mmap(off_t offset, void *bufptr, ...)
>  {
> 
>  ...
>         memcpy(bufptr, info->mmap_buf +
>                (offset - info->mmap_start_offset), read_size);
> 
> 
> Because on s390 copy_to_user() is as fast as userspace memcpy() we
> don't have any benefit here. The only saving is due to less
> mmap()/munmap() than read() system calls because bigger chunks
> are mapped than read.
> 
> If you specify -d 31 the dump memory is fragmented and we have to
> issue more mmap()/munmap() calls and therefore also the system
> call overhead increases.
> 
> If we really want to speed up the mmap path on s390 we probably
> have to get rid of the temporary buffer.
> 
> What do you think?

I'm not sure. Clearly, we should get rid of the temporary buffer. OTOH
this slow-down should be observed on all architectures, not just s390.

Now, mmap should have been implemented in the cache code, not above it.
Since I wrote the cache, this task is probably up to me.

Stay tuned,
Petr T

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