From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ffbox0-bg.mplayerhq.hu (ffbox0-bg.ffmpeg.org [79.124.17.100]) by master.gitmailbox.com (Postfix) with ESMTPS id 371654BA3B for ; Fri, 31 Jan 2025 01:39:52 +0000 (UTC) Received: from [127.0.1.1] (localhost [127.0.0.1]) by ffbox0-bg.mplayerhq.hu (Postfix) with ESMTP id 77B4368BFB8; Fri, 31 Jan 2025 03:39:49 +0200 (EET) Received: from relay3-d.mail.gandi.net (relay3-d.mail.gandi.net [217.70.183.195]) by ffbox0-bg.mplayerhq.hu (Postfix) with ESMTPS id 9EC5768BD17 for ; Fri, 31 Jan 2025 03:39:42 +0200 (EET) Received: by mail.gandi.net (Postfix) with ESMTPSA id 956E2204A9 for ; Fri, 31 Jan 2025 01:39:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=niedermayer.cc; s=gm1; t=1738287581; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=x4uU4SK/NWg7NL4TPrPUES1HJf+XsO502x5iDzOltfs=; b=Rq2lbACmH+Ei/C3X+X42SaqEOh11qWcdZTlbFYSpPaWnGOL5z5MG9Pbd6Kn3ixdwkqw4WN DNhy4GXhKeWPYQW87CGFOScNJdU1KWjGnuTInZ6mR2a+B0GmuuExQh5VH0pJUn/Yd3/WgJ PPtVbnrGQBPw4/L2P7kOPwHwzAxF7T/Bh6J4wn6wEXihujkYQnKOtwpHKTZFcdBL+3y8gN aRHE7rOT25DaH5IFqf7QPLhtYmZGtiHfcDoJatAOKrppKD9gainMS1Ga7oqpxLAfPICyYm nWVRijqztdvWEFBnGctYtk5rd+e6hvj5YOJVcr+rPTzhT9YDgGUJXWC3qOpFzg== Date: Fri, 31 Jan 2025 02:39:40 +0100 From: Michael Niedermayer To: FFmpeg development discussions and patches Message-ID: <20250131013940.GF4991@pb2> References: <20250129095356.2010103-1-martin@martin.st> MIME-Version: 1.0 In-Reply-To: <20250129095356.2010103-1-martin@martin.st> X-GND-State: clean X-GND-Score: -70 X-GND-Cause: gggruggvucftvghtrhhoucdtuddrgeefvddrtddtgdejgedvucetufdoteggodetrfdotffvucfrrhhofhhilhgvmecuifetpfffkfdpucggtfgfnhhsuhgsshgtrhhisggvnecuuegrihhlohhuthemuceftddunecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenfghrlhcuvffnffculdeftddmnecujfgurhepfffhvffukfhfgggtuggjsehgtderredttddunecuhfhrohhmpefoihgthhgrvghlucfpihgvuggvrhhmrgihvghruceomhhitghhrggvlhesnhhivgguvghrmhgrhigvrhdrtggtqeenucggtffrrghtthgvrhhnpedutedvhfduuedugedufefghefhvedvgffgffekhfdvgfdvtefftdejkeehteefheenucfkphepgedurdeiiedrieejrdduudefnecuvehluhhsthgvrhfuihiivgeptdenucfrrghrrghmpehinhgvthepgedurdeiiedrieejrdduudefpdhhvghloheplhhotggrlhhhohhsthdpmhgrihhlfhhrohhmpehmihgthhgrvghlsehnihgvuggvrhhmrgihvghrrdgttgdpnhgspghrtghpthhtohepuddprhgtphhtthhopehffhhmphgvghdquggvvhgvlhesfhhfmhhpvghgrdhorhhg X-GND-Sasl: michael@niedermayer.cc Subject: Re: [FFmpeg-devel] [PATCH] random_seed: Limit the time taken by get_generic_seed X-BeenThere: ffmpeg-devel@ffmpeg.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: FFmpeg development discussions and patches List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: FFmpeg development discussions and patches Content-Type: multipart/mixed; boundary="===============8013586969615828980==" Errors-To: ffmpeg-devel-bounces@ffmpeg.org Sender: "ffmpeg-devel" Archived-At: List-Archive: List-Post: --===============8013586969615828980== Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="oRgRwV0OvsvzOmkZ" Content-Disposition: inline --oRgRwV0OvsvzOmkZ Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Hi Martin On Wed, Jan 29, 2025 at 11:53:53AM +0200, Martin Storsj=F6 wrote: > On a Zen 5, on Ubuntu 24.04 (with CLOCKS_PER_SEC 1000000), the > value of clock() in this loop increments by 0 most of the time, > and when it does increment, it usually increments by 1 compared > to the previous round. >=20 > Due to the "last_t + 2*last_td + (CLOCKS_PER_SEC > 1000) >=3D t" > expression, we only manage to take one step forward in this loop > (incrementing i) if clock() increments by 2, while it incremented > by 0 in the previous iteration (last_td). >=20 > As we do mutate the buffer state even on loop iterations where we > don't increment i, limit the number of times we consecutively can > do this. >=20 > This is similar to the change done in > c4152fc42e480c41efb7f761b1bbe5f0bc43d5bc, to speed it up on > systems with very small CLOCKS_PER_SEC. However in this case, > CLOCKS_PER_SEC is still very large, but the machine is fast enough > to hit every clock increment repeatedly. >=20 > This makes sure that fate-random-seed actually terminates within a > reasonable time on such a system (where it previously could hang, > running for many minutes). > --- > libavutil/random_seed.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) >=20 > diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c > index 8a4e4f1fc0..8f969060a0 100644 > --- a/libavutil/random_seed.c > +++ b/libavutil/random_seed.c > @@ -83,6 +83,7 @@ static uint32_t get_generic_seed(void) > static uint32_t buffer[512] =3D { 0 }; > unsigned char digest[20]; > uint64_t last_i =3D i; > + int cur_iters =3D 0; > =20 > av_assert0(sizeof(tmp) >=3D av_sha_size); > =20 > @@ -98,11 +99,13 @@ static uint32_t get_generic_seed(void) > =20 > for (;;) { > clock_t t =3D clock(); > - if (last_t + 2*last_td + (CLOCKS_PER_SEC > 1000) >=3D t) { > + if (last_t + 2*last_td + (CLOCKS_PER_SEC > 1000) >=3D t && cur_i= ters < 128) { > last_td =3D t - last_t; > buffer[i & 511] =3D 1664525*buffer[i & 511] + 1013904223 + (= last_td % 3294638521U); > + cur_iters++; > } else { > last_td =3D t - last_t; > + cur_iters =3D 0; Iam concerned this could negatively impact entropy The "else" should be run when a interrupt/task switch occured. If that doesnt occur in 128 iterations that doesnt gurantee the entropy has increased. If there are only 0 and 1, ideally we should look at the distribution and go to the else when the pattern differs from the past / has some signs of randomness thx [...] --=20 Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Awnsering whenever a program halts or runs forever is On a turing machine, in general impossible (turings halting problem). On any real computer, always possible as a real computer has a finite number of states N, and will either halt in less than N cycles or never halt. --oRgRwV0OvsvzOmkZ Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iF0EABEKAB0WIQSf8hKLFH72cwut8TNhHseHBAsPqwUCZ5wp2QAKCRBhHseHBAsP q8tBAJ9nNtYDl838n/1c0wKZSGQbhMlidACdGFhsgvjzUYfXKwIzHJ+nj3h63Eg= =js3+ -----END PGP SIGNATURE----- --oRgRwV0OvsvzOmkZ-- --===============8013586969615828980== Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Disposition: inline _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe". --===============8013586969615828980==--