From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CBCAA3B994F for ; Fri, 28 Aug 2026 19:15:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787944545; cv=none; b=deuxY7wJMGGb+x+Nq4u8oGNW4nxKEXxO+3XQIRNK3rXMz/zlQHoDej7ZFvmXFYHMROXRclc2HGx2V5QinEiMjaSRU5j9tzbh1ZSvMcWZpFrX+f4YiS+AiL61AN2B9c0kPV79F7Ut+k25XxZpVqjbAFOByfQe+NaQ3nig2+o4V8s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787944545; c=relaxed/simple; bh=S3nVFmb7RFnAfAS7fYnlT4aMns2DEuHeGK0pmP3ofNU=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=qdbTViXTog7+Y0ee9O07H0nInYMRqQrzf5jdSrKYkr7P73BfE4Z7RvTPhN8CqWxILMitwwqXPM+b64527NYFVPze4u9E4rfSNehj+ZxcLxNeMI1IjCT25+TUppvKVb6JyRP4S4cRlBwTWXPMObz4fyBiUTiOQatjRwJT7KlKXeI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=QofSEutd; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="QofSEutd" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CDDF51F000E9; Fri, 28 Aug 2026 19:15:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787944543; bh=S3nVFmb7RFnAfAS7fYnlT4aMns2DEuHeGK0pmP3ofNU=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=QofSEutd+NHg7e6UlCIPEfNv+5AbrObTVOZa4qOQ3eFGPHmZJyjTHgD+fJwq6jnug k6yB4AYEl9L6DnCembOWa6xGgqUQEcYyMYLcPZtJncNjTYNxd85l2qhYqT6ENQTyel 5DgEDMJxPph6iptI8VcOcTSYhYRI1wonCrCOwRg823q7sV2SBlWxSewbNTBRB9Tnvb V+/sDGIzXiCZ3eB5rBzfsRonM36iZIcGqYPHPXGYmK27a9EZBuOAPCSCg4c9Qbj0Fv PEm4ro5ssaBg0xJjZtXEVfgzuzZO79xglqJkCXwO27Bcv3aRMJvSFXFpDWH2ljIFUr D954bp6mk8FYQ== Message-ID: Subject: Re: [PATCH v2 5/7] NFSD: Evict completed DRC entries via implied ACK From: Jeff Layton To: Chuck Lever , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey Cc: Rick Macklem , linux-nfs@vger.kernel.org Date: Fri, 28 Aug 2026 15:15:41 -0400 In-Reply-To: <10c97c34-1061-4aa2-a5b5-234e4d01e894@app.fastmail.com> References: <20260828-duplicate-reply-cache-v2-0-25069e660a7b@kernel.org> <20260828-duplicate-reply-cache-v2-5-25069e660a7b@kernel.org> <85acfa01848e5d7829ca18ad8864410fbb5e12b7.camel@kernel.org> <10c97c34-1061-4aa2-a5b5-234e4d01e894@app.fastmail.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2026-08-28 at 14:59 -0400, Chuck Lever wrote: > On Fri, Aug 28, 2026, at 2:38 PM, Jeff Layton wrote: > > On Fri, 2026-08-28 at 12:17 -0400, Chuck Lever wrote: > > > A completed DRC entry stays in its bucket until RC_EXPIRE elapses or > > > the cache exceeds max_drc_entries. Entries whose replies the client > > > already holds lengthen the bucket and slow every lookup that hashes > > > there. > > >=20 > > > RFC 1813 Section 4.5 observes that on a connection-oriented transport > > > a duplicate request arises from reconnection, not from within a live > > > connection. A fresh request on a live TCP or RDMA connection therefor= e > > > means the client is not retransmitting an earlier one. UDP clients > > > retransmit on timeout over a shared svc_xprt, so eviction is > > > restricted to transports marked XPT_ORDERED. Even there the evidence > > > is not conclusive, since a client with several requests outstanding > > > sends the next before the previous reply arrives. The cache is > > > advisory: a premature eviction costs a miss and re-execution, the sam= e > > > outcome memory pressure and RC_EXPIRE already produce. > >=20 > > No, it's not. The DRC is necessary for proper function, and if we drop > > non-idempotent requests prematurely, then that could cause spurious > > errors. > >=20 > > Or am I misunderstanding what you mean by "The cache" here? >=20 > Not talking about the NFSv4.1 session cache. The old DRC has > always been best-effort. It cannot be relied upon for proper > function, since the DRC is non-deterministic. >=20 Yes, but in this case you can potentially evict this thing well before you actually need it: - client does a v3 RENAME (anything non-idempotent, really) - reply to the RENAME goes out just before a new call comes in (WRITE or something maybe). The RENAME DRC entry's timestamp is a few jiffies earlier so we evict it. - connection drops before client gets the reply and client reconnects and retransmits the v3 RENAME ...hilarity ensues? Maybe this sort of bad luck never happens in practice, but I don't see what would prevent it. > The point I'm making here is that items already get evicted > due to memory pressure or because the DRC size is capped. >=20 > In particular, if you are actively removing items that are > very unlikely to be hit, that *reduces* the likelihood that > other, perhaps more valuable, items will be evicted due to > memory pressure. >=20 Sure, but usually we evict the oldest stuff first, and we generally keep a big enough cache to get past the point where retransmissions occur. This patchset changes that and will evict things earlier. Maybe that's OK in practice. --=20 Jeff Layton