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bpo-36051: Drop GIL during large bytes.join() (GH-17757)

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Improve multi-threaded performance by dropping the GIL in the fast path
of bytes.join. To avoid increasing overhead for small joins, it is only
done if the output size exceeds a threshold.
This commit is contained in:
Bruce Merry 2020-01-29 09:09:24 +02:00 • committed by GitHub
parent 6a65eba44b
commit d07d9f4c43
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GPG key ID: 4AEE18F83AFDEB23
4 changed files with 48 additions and 19 deletions
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57
Objects/stringlib/join.h
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@ -18,6 +18,9 @@ STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
Py_buffer *buffers = NULL;
#define NB_STATIC_BUFFERS 10
Py_buffer static_buffers[NB_STATIC_BUFFERS];
#define GIL_THRESHOLD 1048576
int drop_gil = 1;
PyThreadState *save;
seq = PySequence_Fast(iterable, "can only join an iterable");
if (seq == NULL) {
@ -65,12 +68,21 @@ STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
buffers[i].buf = PyBytes_AS_STRING(item);
buffers[i].len = PyBytes_GET_SIZE(item);
}
else if (PyObject_GetBuffer(item, &buffers[i], PyBUF_SIMPLE) != 0) {
PyErr_Format(PyExc_TypeError,
"sequence item %zd: expected a bytes-like object, "
"%.80s found",
i, Py_TYPE(item)->tp_name);
goto error;
else {
if (PyObject_GetBuffer(item, &buffers[i], PyBUF_SIMPLE) != 0) {
PyErr_Format(PyExc_TypeError,
"sequence item %zd: expected a bytes-like object, "
"%.80s found",
i, Py_TYPE(item)->tp_name);
goto error;
}
/* If the backing objects are mutable, then dropping the GIL
* opens up race conditions where another thread tries to modify
* the object which we hold a buffer on it. Such code has data
* races anyway, but this is a conservative approach that avoids
* changing the behaviour of that data race.
*/
drop_gil = 0;
}
nbufs = i + 1; /* for error cleanup */
itemlen = buffers[i].len;
@ -102,6 +114,12 @@ STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
/* Catenate everything. */
p = STRINGLIB_STR(res);
if (sz < GIL_THRESHOLD) {
drop_gil = 0; /* Benefits are likely outweighed by the overheads */
}
if (drop_gil) {
save = PyEval_SaveThread();
}
if (!seplen) {
/* fast path */
for (i = 0; i < nbufs; i++) {
@ -110,19 +128,23 @@ STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
memcpy(p, q, n);
p += n;
}
goto done;
}
for (i = 0; i < nbufs; i++) {
Py_ssize_t n;
char *q;
if (i) {
memcpy(p, sepstr, seplen);
p += seplen;
else {
for (i = 0; i < nbufs; i++) {
Py_ssize_t n;
char *q;
if (i) {
memcpy(p, sepstr, seplen);
p += seplen;
}
n = buffers[i].len;
q = buffers[i].buf;
memcpy(p, q, n);
p += n;
}
n = buffers[i].len;
q = buffers[i].buf;
memcpy(p, q, n);
p += n;
}
if (drop_gil) {
PyEval_RestoreThread(save);
}
goto done;
@ -138,3 +160,4 @@ done:
}
#undef NB_STATIC_BUFFERS
#undef GIL_THRESHOLD