Multi-conn is enabled only when we know the connection is read-only:
$ ./nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run '
nbdinfo $uri ' | grep can_multi_conn
can_multi_conn: true
$ ./nbdkit curl file:/var/tmp/jammy-server-cloudimg-amd64.raw --run ' nbdinfo
$uri ' | grep can_multi_conn
can_multi_conn: false
This improves performance.
Comparing before and after this commit shows approximate doubling of
performance:
Benchmark 1: nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw
--run "nbdcopy -p \$uri null:"
Time (mean ± σ): 943.8 ms ± 18.8 ms [User: 316.2 ms, System: 1029.7
ms]
Range (min … max): 923.7 ms … 989.2 ms 10 runs
Benchmark 2: ~/d/nbdkit/nbdkit -r curl
file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:"
Time (mean ± σ): 455.0 ms ± 6.2 ms [User: 542.2 ms, System: 1824.7
ms]
Range (min … max): 449.1 ms … 471.6 ms 10 runs
Summary
' ~/d/nbdkit/nbdkit -r curl file:/var/tmp/jammy-server-cloudimg-amd64.raw
--run "nbdcopy -p \$uri null:" ' ran
2.07 ± 0.05 times faster than ' nbdkit -r curl
file:/var/tmp/jammy-server-cloudimg-amd64.raw --run "nbdcopy -p \$uri null:" '
See also:
https://listman.redhat.com/archives/libguestfs/2023-February/030581.html
---
plugins/curl/curldefs.h | 1 +
plugins/curl/curl.c | 14 ++++++++++++++
2 files changed, 15 insertions(+)
diff --git a/plugins/curl/curldefs.h b/plugins/curl/curldefs.h
index a2c521c50..16b6071cc 100644
--- a/plugins/curl/curldefs.h
+++ b/plugins/curl/curldefs.h
@@ -68,6 +68,7 @@ extern const char *user_agent;
/* The per-connection handle. */
struct curl_handle {
CURL *c;
+ int readonly;
bool accept_range;
int64_t exportsize;
char errbuf[CURL_ERROR_SIZE];
diff --git a/plugins/curl/curl.c b/plugins/curl/curl.c
index 42aba4216..2f34d1dc1 100644
--- a/plugins/curl/curl.c
+++ b/plugins/curl/curl.c
@@ -465,6 +465,7 @@ curl_open (int readonly)
nbdkit_error ("calloc: %m");
return NULL;
}
+ h->readonly = readonly;
h->c = curl_easy_init ();
if (h->c == NULL) {
@@ -781,6 +782,18 @@ curl_get_size (void *handle)
return h->exportsize;
}
+/* Multi-conn is safe for read-only connections, but HTTP does not
+ * have any concept of flushing so we cannot use it for read-write
+ * connections.
+ */
+static int
+curl_can_multi_conn (void *handle)
+{
+ struct curl_handle *h = handle;
+
+ return !! h->readonly;
+}
+
/* NB: The terminology used by libcurl is confusing!
*
* WRITEFUNCTION / write_cb is used when reading from the remote server
@@ -924,6 +937,7 @@ static struct nbdkit_plugin plugin = {
.open = curl_open,
.close = curl_close,
.get_size = curl_get_size,
+ .can_multi_conn = curl_can_multi_conn,
.pread = curl_pread,
.pwrite = curl_pwrite,
};
--
2.39.0
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