py/vstr: Change allocation policy, +16 to requested size, instead of *2.

Effect measured on esp8266 port:

Before:
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44214 ms
This machine benchmarks at 226 pystones/second
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44246 ms
This machine benchmarks at 226 pystones/second

After:
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44343ms
This machine benchmarks at 225 pystones/second
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44376ms
This machine benchmarks at 225 pystones/second

1 files changed, 1 insertions, 1 deletions

diff --git a/py/vstr.c b/py/vstr.c
index f558e3fcb1..cf10f8471a 100644
--- a/py/vstr.c
+++ b/py/vstr.c
@@ -151,7 +151,7 @@ STATIC bool vstr_ensure_extra(vstr_t *vstr, size_t size) {
         if (vstr->fixed_buf) {
             return false;
         }
-        size_t new_alloc = ROUND_ALLOC((vstr->len + size) * 2);
+        size_t new_alloc = ROUND_ALLOC((vstr->len + size) + 16);
         char *new_buf = m_renew(char, vstr->buf, vstr->alloc, new_alloc);
         if (new_buf == NULL) {
             vstr->had_error = true;