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

[Michael Holzheu <holzheu@linux.vnet.ibm.com>]

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?
Michael


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