From mboxrd@z Thu Jan 1 00:00:00 1970 Received: by passt.top (Postfix, from userid 1000) id 53BCF5A0624; Mon, 08 Dec 2025 01:22:29 +0100 (CET) From: Stefano Brivio To: passt-dev@passt.top Subject: [PATCH v2 1/9] tcp, util: Add function for scaling to linearly interpolated factor, use it Date: Mon, 8 Dec 2025 01:22:09 +0100 Message-ID: <20251208002229.391162-2-sbrivio@redhat.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251208002229.391162-1-sbrivio@redhat.com> References: <20251208002229.391162-1-sbrivio@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Message-ID-Hash: ZUI2MZHYFV6WQVKFMXX7PCIPQF3YDWYU X-Message-ID-Hash: ZUI2MZHYFV6WQVKFMXX7PCIPQF3YDWYU X-MailFrom: sbrivio@passt.top X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: Max Chernoff , David Gibson X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: Right now, the only need for this kind of function comes from tcp_get_sndbuf(), which calculates the amount of sending buffer we want to use depending on its own size: we want to use more of it if it's smaller, as bookkeeping overhead is usually lower and we rely on auto-tuning there, and use less of it when it's bigger. For this purpose, the new function is overly generic and its name is a mouthful: @x is the same as @y, that is, we want to use more or less of the buffer depending on the size of the buffer itself. However, an upcoming change will need that generality, as we'll want to scale the amount of sending buffer we use depending on another (scaled) factor. While at it, now that we have this new function, which makes it simple to specify a precise usage factor, change the amount of sending buffer we want to use at and above 4 MiB: 75% looks perfectly safe. Signed-off-by: Stefano Brivio --- tcp.c | 8 ++------ util.c | 38 ++++++++++++++++++++++++++++++++++++++ util.h | 1 + 3 files changed, 41 insertions(+), 6 deletions(-) diff --git a/tcp.c b/tcp.c index bb661ee..37012cc 100644 --- a/tcp.c +++ b/tcp.c @@ -773,7 +773,7 @@ static void tcp_rtt_dst_check(const struct tcp_tap_conn *conn, } /** - * tcp_get_sndbuf() - Get, scale SO_SNDBUF between thresholds (1 to 0.5 usage) + * tcp_get_sndbuf() - Get, scale SO_SNDBUF between thresholds (1 to 0.75 usage) * @conn: Connection pointer */ static void tcp_get_sndbuf(struct tcp_tap_conn *conn) @@ -788,11 +788,7 @@ static void tcp_get_sndbuf(struct tcp_tap_conn *conn) return; } - v = sndbuf; - if (v >= SNDBUF_BIG) - v /= 2; - else if (v > SNDBUF_SMALL) - v -= v * (v - SNDBUF_SMALL) / (SNDBUF_BIG - SNDBUF_SMALL) / 2; + v = scale_x_to_y_slope(sndbuf, sndbuf, SNDBUF_SMALL, SNDBUF_BIG, 75); SNDBUF_SET(conn, MIN(INT_MAX, v)); } diff --git a/util.c b/util.c index f32c9cb..ff0ba01 100644 --- a/util.c +++ b/util.c @@ -1223,3 +1223,41 @@ void fsync_pcap_and_log(void) if (log_file != -1) (void)fsync(log_file); } + +/** + * scale_x_to_y_slope() - Scale @x from 100% to f% depending on @y's value + * @x: Value to scale + * @y: Value determining scaling + * @lo: Lower bound for @y (start of y-axis slope) + * @hi: Upper bound for @y (end of y-axis slope) + * @f: Scaling factor, percent + * + * Return: @x scaled by @f * linear interpolation of @y between @lo and @hi + * + * In pictures: + * + * f % -> ,---- * If @y < lo (for example, @y is y0), return @x + * /| | + * / | | * If @lo < @y < @hi (for example, @y is y1), + * / | | return @x scaled by a factor linearly + * (100 + f) / 2 % ->/ | | interpolated between 100% and f% depending on + * /| | | @y's position between @lo (100%) and @hi (f%) + * / | | | + * / | | | * If @y > @hi (for example, @y is y2), return + * 100 % -> -----' | | | @x * @f / 100 + * | | | | | + * y0 lo y1 hi y2 Example: @f = 150, @lo = 10, @hi = 20, @y = 15, + * @x = 1000 + * -> interpolated factor is 125% + * -> return 1250 + */ +long scale_x_to_y_slope(long x, long y, long lo, long hi, long f) +{ + if (y < lo) + return x; + + if (y > hi) + return x * f / 100; + + return x - (x * (y - lo) / (hi - lo)) * (100 - f) / 100; +} diff --git a/util.h b/util.h index 17f5ae0..ec75453 100644 --- a/util.h +++ b/util.h @@ -242,6 +242,7 @@ int read_remainder(int fd, const struct iovec *iov, size_t cnt, size_t skip); void close_open_files(int argc, char **argv); bool snprintf_check(char *str, size_t size, const char *format, ...); void fsync_pcap_and_log(void); +long scale_x_to_y_slope(long x, long y, long lo, long hi, long f); /** * af_name() - Return name of an address family -- 2.43.0