stefan2_at_apache.org wrote on Wed, Oct 27, 2010 at 20:47:06 -0000:
> Author: stefan2
> Date: Wed Oct 27 20:47:06 2010
> New Revision: 1028094
>
> URL: http://svn.apache.org/viewvc?rev=1028094&view=rev
> Log:
> If we allocate stringbufs, their actual capacity allocated by APR
> is often larger than requested. Take advantage of that by requesting
> an enlarged buffer in the first place. Especially for short or empty
> strings, this saves some re-allocations once they grow.
>
> * subversion/libsvn_subr/svn_string.c
> (svn_stringbuf_ncreate): allocate a buffer of aligned size
>
> Modified:
> subversion/branches/performance/subversion/libsvn_subr/svn_string.c
>
> Modified: subversion/branches/performance/subversion/libsvn_subr/svn_string.c
> URL: http://svn.apache.org/viewvc/subversion/branches/performance/subversion/libsvn_subr/svn_string.c?rev=1028094&r1=1028093&r2=1028094&view=diff
> ==============================================================================
> --- subversion/branches/performance/subversion/libsvn_subr/svn_string.c (original)
> +++ subversion/branches/performance/subversion/libsvn_subr/svn_string.c Wed Oct 27 20:47:06 2010
> @@ -266,9 +266,21 @@ svn_stringbuf_create_ensure(apr_size_t b
> svn_stringbuf_t *
> svn_stringbuf_ncreate(const char *bytes, apr_size_t size, apr_pool_t *pool)
> {
> - /* Ensure string buffer of size + 1 */
> - svn_stringbuf_t *strbuf = svn_stringbuf_create_ensure(size, pool);
> + char *data;
>
Is this new variable used anywhere?
> + /* apr_palloc will allocate multiples of 8.
> + * Thus, we would waste some of that memory if we stuck to the
> + * smaller size. Note that this is safe even if apr_palloc would
> + * use some other aligment or none at all. */
> + apr_size_t aligned_size = APR_ALIGN_DEFAULT(size + 1) - 1;
> +
So, this rounds the requested number of bytes (size+1) to the next
multiple of 8, then subtracts 1 from the result? Net effect being that
if we need 1 byte, we get 7; and if we need 2 or 7 bytes, then we get
7 bytes, right?
But if we need size+1=8 bytes, then.. I think we get aligned_size=7, and
svn_stringbuf_create_ensure() compensates because it allocates one byte
more than requested, right?
> + /* Ensure string buffer of aligned_size + 1.
> + * This should allocate the same amount of memory as "size" would. */
> + svn_stringbuf_t *strbuf = svn_stringbuf_create_ensure(aligned_size, pool);
> +
Could this "use aligned_size instead of size" logic be pushed down to
svn_stringbuf_create_ensure()?
> + /* Actually, aligned_size+1 would be faster but we cannot be entirely
> + * sure that the source string has been aligned properly such that
> + * all the extra bytes would actually come from addressible memory.*/
> memcpy(strbuf->data, bytes, size);
>
> /* Null termination is the convention -- even if we suspect the data
>
>
Received on 2010-10-28 14:33:16 CEST