[Qemu-devel] Re: [PATCH] qemu-kvm: Speed up of the dirty-bitmap-traveling

[Jan Kiszka <jan.kiszka@siemens.com>]

OHMURA Kei wrote:
>>> Would be great if you could provide a version for upstream as well
>>> because it will likely replace this qemu-kvm code on day.
>> O.K.  We'll prepare it.
> 
> 
> We have implemented the version for upstream.  Some source code are borrowed 
> from qemu-kvm.c.  It is not fully tested yet, though.
> 
> We also did performance test against this patch.  Test environment is the same 
> as the email I sent before.
> 
> 
> Experimental results:
> Test1: Guest OS read 3GB file, which is bigger than memory.
> #called     orig.(msec)     patch(msec)     ratio
> 14          3.79            0.18            20.8
> 12          3.20            0.15            21.4
> 11          2.89            0.14            21.0
>  
> Test2: Guest OS read/write 3GB file, which is bigger than memory.
> #called     orig.(msec)     patch(msec)     ratio
> 364         180             8.70            20.7
> 326         161             7.71            20.9
> 474         235             11.7            20.1
> 

Wow, so we were really inefficient here. Nice work!

Once you are done with your tests, please post this against
qemu-kvm.git's uq/master so that Avi or Marcelo can push it upstream.
Minor remarks below.

> 
> ---
>  kvm-all.c |   80 +++++++++++++++++++++++++++++++++++++++++++++++++-----------
>  1 files changed, 65 insertions(+), 15 deletions(-)
> 
> diff --git a/kvm-all.c b/kvm-all.c
> index 15ec38e..9666843 100644
> --- a/kvm-all.c
> +++ b/kvm-all.c
> @@ -279,9 +279,69 @@ int kvm_set_migration_log(int enable)
>      return 0;
>  }
>  
> -static int test_le_bit(unsigned long nr, unsigned char *addr)
> +static inline void kvm_get_dirty_pages_log_range_by_byte(unsigned int start,

I don't think inline is appropriate here. Smart compilers are able to do
this on their own. And small code footprint actually contributes to
speed as well.

> +                                                         unsigned int end,
> +                                                         unsigned char *bitmap,
> +                                                         unsigned long offset)
>  {
> -    return (addr[nr >> 3] >> (nr & 7)) & 1;
> +    unsigned int i, j, n = 0;
> +    unsigned long page_number, addr, addr1;
> +    ram_addr_t ram_addr;
> +    unsigned char c;
> +
> +    /*   
> +     * bitmap-traveling is faster than memory-traveling (for addr...)
> +     * especially when most of the memory is not dirty.
> +     */
> +    for (i = start; i < end; i++) {
> +        c = bitmap[i];
> +        while (c > 0) {
> +            j = ffsl(c) - 1;
> +            c &= ~(1u << j);
> +            page_number = i * 8 + j;
> +            addr1 = page_number * TARGET_PAGE_SIZE;
> +            addr = offset + addr1;
> +            ram_addr = cpu_get_physical_page_desc(addr);
> +            cpu_physical_memory_set_dirty(ram_addr);
> +            n++;
> +        }
> +    }
> +}
> +
> +static int kvm_get_dirty_pages_log_range_by_long(unsigned long start_addr,
> +                                                 unsigned char *bitmap,
> +                                                 unsigned long mem_size)
> +{
> +    unsigned int i;
> +    unsigned int len;
> +    unsigned long *bitmap_ul = (unsigned long *)bitmap;
> +
> +    /* bitmap-traveling by long size is faster than by byte size
> +     * especially when most of memory is not dirty.
> +     * bitmap should be long-size aligned for traveling by long.
> +     */
> +    if (((unsigned long)bitmap & (TARGET_LONG_SIZE - 1)) == 0) {
> +        len = ((mem_size / TARGET_PAGE_SIZE) + TARGET_LONG_BITS - 1) / 
> +            TARGET_LONG_BITS;
> +        for (i = 0; i < len; i++)
> +            if (bitmap_ul[i] != 0)
> +                kvm_get_dirty_pages_log_range_by_byte(i * TARGET_LONG_SIZE,
> +                    (i + 1) * TARGET_LONG_SIZE, bitmap, start_addr);

Missing { }, 2x.

> +        /*                                                      
> +         * We will check the remaining dirty-bitmap, 
> +         * when the mem_size is not a multiple of TARGET_LONG_SIZE. 
> +         */
> +        if ((mem_size & (TARGET_LONG_SIZE - 1)) != 0) {
> +            len = ((mem_size / TARGET_PAGE_SIZE) + 7) / 8;
> +            kvm_get_dirty_pages_log_range_by_byte(i * TARGET_LONG_SIZE,
> +                len, bitmap, start_addr);

This line should be indented to the '('.

> +        }
> +    } else { /* slow path: traveling by byte. */
> +        len = ((mem_size / TARGET_PAGE_SIZE) + 7) / 8;
> +        kvm_get_dirty_pages_log_range_by_byte(0, len, bitmap, start_addr);
> +    }
> +
> +    return 0;
>  }
>  
>  /**
> @@ -297,8 +357,6 @@ int kvm_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
>  {
>      KVMState *s = kvm_state;
>      unsigned long size, allocated_size = 0;
> -    target_phys_addr_t phys_addr;
> -    ram_addr_t addr;
>      KVMDirtyLog d;
>      KVMSlot *mem;
>      int ret = 0;
> @@ -327,17 +385,9 @@ int kvm_physical_sync_dirty_bitmap(target_phys_addr_t start_addr,
>              break;
>          }
>  
> -        for (phys_addr = mem->start_addr, addr = mem->phys_offset;
> -             phys_addr < mem->start_addr + mem->memory_size;
> -             phys_addr += TARGET_PAGE_SIZE, addr += TARGET_PAGE_SIZE) {
> -            unsigned char *bitmap = (unsigned char *)d.dirty_bitmap;
> -            unsigned nr = (phys_addr - mem->start_addr) >> TARGET_PAGE_BITS;
> -
> -            if (test_le_bit(nr, bitmap)) {
> -                cpu_physical_memory_set_dirty(addr);
> -            }
> -        }
> -        start_addr = phys_addr;
> +        kvm_get_dirty_pages_log_range_by_long(mem->start_addr, 
> +            d.dirty_bitmap, mem->memory_size);
> +        start_addr = mem->start_addr + mem->memory_size;
>      }
>      qemu_free(d.dirty_bitmap);
>  

Thanks,
Jan

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