[Libguestfs] [nbdkit PATCH] data, memory: Optimize .zero > PAGE_SIZE

[Eric Blake]

On 8/15/19 5:25 AM, Richard W.M. Jones wrote:
> On Wed, Aug 14, 2019 at 09:10:15PM -0500, Eric Blake wrote:
>> When sparse_array_zero() is used for a range larger than a page,
>> there's no need to waste time in memset() or is_zero() - we already
>> know the page will be free()d.
>>
>> Signed-off-by: Eric Blake <eblake at redhat.com>
>> ---
>>
>> Here's a fun one :)
>>
>>  common/sparse/sparse.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/common/sparse/sparse.c b/common/sparse/sparse.c
>> index cb44743c..5e085763 100644
>> --- a/common/sparse/sparse.c
>> +++ b/common/sparse/sparse.c
>> @@ -343,10 +343,13 @@ sparse_array_zero (struct sparse_array *sa, uint32_t count, uint64_t offset)
>>        n = count;
>>
>>      if (p) {
>> -      memset (p, 0, n);
>> +      if (n < PAGE_SIZE)
>> +        memset (p, 0, n);
>> +      else
>> +        assert (p == *l2_page);
>>
>>        /* If the whole page is now zero, free it. */
>> -      if (is_zero (*l2_page, PAGE_SIZE)) {
>> +      if (n == PAGE_SIZE || is_zero (*l2_page, PAGE_SIZE)) {
> 
> Should this be n >= PAGE_SIZE?  My guess is no because lookup (..&n..)
> can't return n larger than the remaining size of the page
> (ie. n <= PAGE_SIZE).

Correct, but that's action at a distance; using >= doesn't change
semantics and brings the action closer to the code. I'll make that tweak.

> 
>>          if (sa->debug)
>>            nbdkit_debug ("%s: freeing zero page at offset %" PRIu64,
>>                          __func__, offset);
> 
> Anyway, ACK.
> 
> Rich.
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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