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[Qemu-devel] [PATCH 2/2] find_ram_offset: Align ram_addr_t allocation on long boundaries

Dr. David Alan Gilbert (git) posted 2 patches 7 years, 9 months ago
There is a newer version of this series
[Qemu-devel] [PATCH 2/2] find_ram_offset: Align ram_addr_t allocation on long boundaries
Posted by Dr. David Alan Gilbert (git) 7 years, 9 months ago 
From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

The dirty bitmaps are built from 'long'sand there is fast-path code
for synchronising the case where the RAMBlock is aligned to the start
of a long boundary.  Align the allocation to this boundary
to cause the fast path to be used.

Offsets before change:
11398@1515169675.018566:find_ram_offset size: 0x1e0000 @ 0x8000000
11398@1515169675.020064:find_ram_offset size: 0x20000 @ 0x81e0000
11398@1515169675.020244:find_ram_offset size: 0x20000 @ 0x8200000
11398@1515169675.024343:find_ram_offset size: 0x1000000 @ 0x8220000
11398@1515169675.025154:find_ram_offset size: 0x10000 @ 0x9220000
11398@1515169675.027682:find_ram_offset size: 0x40000 @ 0x9230000
11398@1515169675.032921:find_ram_offset size: 0x200000 @ 0x9270000
11398@1515169675.033307:find_ram_offset size: 0x1000 @ 0x9470000
11398@1515169675.033601:find_ram_offset size: 0x1000 @ 0x9471000

after change:
10923@1515169108.818245:find_ram_offset size: 0x1e0000 @ 0x8000000
10923@1515169108.819410:find_ram_offset size: 0x20000 @ 0x8200000
10923@1515169108.819587:find_ram_offset size: 0x20000 @ 0x8240000
10923@1515169108.823708:find_ram_offset size: 0x1000000 @ 0x8280000
10923@1515169108.824503:find_ram_offset size: 0x10000 @ 0x9280000
10923@1515169108.827093:find_ram_offset size: 0x40000 @ 0x92c0000
10923@1515169108.833045:find_ram_offset size: 0x200000 @ 0x9300000
10923@1515169108.833504:find_ram_offset size: 0x1000 @ 0x9500000
10923@1515169108.833787:find_ram_offset size: 0x1000 @ 0x9540000

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 exec.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/exec.c b/exec.c
index 7966570231..644603f05e 100644
--- a/exec.c
+++ b/exec.c
@@ -1694,6 +1694,11 @@ static ram_addr_t find_ram_offset(ram_addr_t size)
             }
         }
 
+        /* Align blocks to start on a 'long' in the bitmap
+         * which makes the bitmap sync'ing take the fast path.
+         */
+        end = ROUND_UP(end, BITS_PER_LONG << TARGET_PAGE_BITS);
+
         /* If it fits remember our place and remember the size
          * of gap, but keep going so that we might find a smaller
          * gap to fill so avoiding fragmentation.
-- 
2.14.3
Re: [Qemu-devel] [PATCH 2/2] find_ram_offset: Align ram_addr_t allocation on long boundaries
Posted by Eric Blake 7 years, 9 months ago 
On 01/05/2018 11:01 AM, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> The dirty bitmaps are built from 'long'sand there is fast-path code

s/'long'sand/'long's, and/

> for synchronising the case where the RAMBlock is aligned to the start
> of a long boundary.  Align the allocation to this boundary
> to cause the fast path to be used.
> 

> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  exec.c | 5 +++++
>  1 file changed, 5 insertions(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/exec.c b/exec.c
> index 7966570231..644603f05e 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1694,6 +1694,11 @@ static ram_addr_t find_ram_offset(ram_addr_t size)
>              }
>          }
>  
> +        /* Align blocks to start on a 'long' in the bitmap
> +         * which makes the bitmap sync'ing take the fast path.
> +         */
> +        end = ROUND_UP(end, BITS_PER_LONG << TARGET_PAGE_BITS);
> +
>          /* If it fits remember our place and remember the size
>           * of gap, but keep going so that we might find a smaller
>           * gap to fill so avoiding fragmentation.
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org
Re: [Qemu-devel] [PATCH 2/2] find_ram_offset: Align ram_addr_t allocation on long boundaries
Posted by Juan Quintela 7 years, 9 months ago 
"Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
>
> The dirty bitmaps are built from 'long'sand there is fast-path code
> for synchronising the case where the RAMBlock is aligned to the start
> of a long boundary.  Align the allocation to this boundary
> to cause the fast path to be used.
>
> Offsets before change:
> 11398@1515169675.018566:find_ram_offset size: 0x1e0000 @ 0x8000000
> 11398@1515169675.020064:find_ram_offset size: 0x20000 @ 0x81e0000
> 11398@1515169675.020244:find_ram_offset size: 0x20000 @ 0x8200000
> 11398@1515169675.024343:find_ram_offset size: 0x1000000 @ 0x8220000
> 11398@1515169675.025154:find_ram_offset size: 0x10000 @ 0x9220000
> 11398@1515169675.027682:find_ram_offset size: 0x40000 @ 0x9230000
> 11398@1515169675.032921:find_ram_offset size: 0x200000 @ 0x9270000
> 11398@1515169675.033307:find_ram_offset size: 0x1000 @ 0x9470000
> 11398@1515169675.033601:find_ram_offset size: 0x1000 @ 0x9471000
>
> after change:
> 10923@1515169108.818245:find_ram_offset size: 0x1e0000 @ 0x8000000
> 10923@1515169108.819410:find_ram_offset size: 0x20000 @ 0x8200000
> 10923@1515169108.819587:find_ram_offset size: 0x20000 @ 0x8240000
> 10923@1515169108.823708:find_ram_offset size: 0x1000000 @ 0x8280000
> 10923@1515169108.824503:find_ram_offset size: 0x10000 @ 0x9280000
> 10923@1515169108.827093:find_ram_offset size: 0x40000 @ 0x92c0000
> 10923@1515169108.833045:find_ram_offset size: 0x200000 @ 0x9300000
> 10923@1515169108.833504:find_ram_offset size: 0x1000 @ 0x9500000
> 10923@1515169108.833787:find_ram_offset size: 0x1000 @ 0x9540000
>
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>
Re: [Qemu-devel] [PATCH 2/2] find_ram_offset: Align ram_addr_t allocation on long boundaries
Posted by Paolo Bonzini 7 years, 9 months ago 
On 05/01/2018 18:01, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> The dirty bitmaps are built from 'long'sand there is fast-path code
> for synchronising the case where the RAMBlock is aligned to the start
> of a long boundary.  Align the allocation to this boundary
> to cause the fast path to be used.
> 
> Offsets before change:
> 11398@1515169675.018566:find_ram_offset size: 0x1e0000 @ 0x8000000
> 11398@1515169675.020064:find_ram_offset size: 0x20000 @ 0x81e0000
> 11398@1515169675.020244:find_ram_offset size: 0x20000 @ 0x8200000
> 11398@1515169675.024343:find_ram_offset size: 0x1000000 @ 0x8220000
> 11398@1515169675.025154:find_ram_offset size: 0x10000 @ 0x9220000
> 11398@1515169675.027682:find_ram_offset size: 0x40000 @ 0x9230000
> 11398@1515169675.032921:find_ram_offset size: 0x200000 @ 0x9270000
> 11398@1515169675.033307:find_ram_offset size: 0x1000 @ 0x9470000
> 11398@1515169675.033601:find_ram_offset size: 0x1000 @ 0x9471000
> 
> after change:
> 10923@1515169108.818245:find_ram_offset size: 0x1e0000 @ 0x8000000
> 10923@1515169108.819410:find_ram_offset size: 0x20000 @ 0x8200000
> 10923@1515169108.819587:find_ram_offset size: 0x20000 @ 0x8240000
> 10923@1515169108.823708:find_ram_offset size: 0x1000000 @ 0x8280000
> 10923@1515169108.824503:find_ram_offset size: 0x10000 @ 0x9280000
> 10923@1515169108.827093:find_ram_offset size: 0x40000 @ 0x92c0000
> 10923@1515169108.833045:find_ram_offset size: 0x200000 @ 0x9300000
> 10923@1515169108.833504:find_ram_offset size: 0x1000 @ 0x9500000
> 10923@1515169108.833787:find_ram_offset size: 0x1000 @ 0x9540000
> 
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  exec.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/exec.c b/exec.c
> index 7966570231..644603f05e 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1694,6 +1694,11 @@ static ram_addr_t find_ram_offset(ram_addr_t size)
>              }
>          }
>  
> +        /* Align blocks to start on a 'long' in the bitmap
> +         * which makes the bitmap sync'ing take the fast path.
> +         */
> +        end = ROUND_UP(end, BITS_PER_LONG << TARGET_PAGE_BITS);

I think this should be done before the nested loop.  Otherwise, "next -
end" can become negative, which is always going to be >= size.  It's
also very large, so it's likely to fail the mingap test, but it's buggy
nevertheless.

In fact "end" could be renamed to "candidate" which explains more
clearly what's going on.  I'll post a v2 shortly...

Thanks,

Paolo


>          /* If it fits remember our place and remember the size
>           * of gap, but keep going so that we might find a smaller
>           * gap to fill so avoiding fragmentation.
>

Patchew 2.1-devel-b67e4c2

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