From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id AF806CDB474 for ; Fri, 20 Oct 2023 17:09:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229683AbjJTRJP (ORCPT ); Fri, 20 Oct 2023 13:09:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44450 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229522AbjJTRJO (ORCPT ); Fri, 20 Oct 2023 13:09:14 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4B21BCA for ; Fri, 20 Oct 2023 10:09:13 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4B487C433C7; Fri, 20 Oct 2023 17:09:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1697821752; bh=eWD+Adzj/v2h0Z2nAsdfUjQtNIPGZlsELZD/XCkarcI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ayxwV5WPb0DpgRE7P9klWfunOOKJkgPKNs+gqYlWZTyOwCXGF0frMT6MITkM5UBNO ydS6ib980YAlmuJBCZYEywyrFUrjd+YUvogZzX4NbCbhmJWsqVvVcxTPXuDNQf4RCh gz+KPVLKpQn+TEZN6/7CxR6wbBRcRYxShcY2uq1W5EBb6Y8XgP72pBLV+AUZB4FwnM mqaxB3jmNT7OLmwhDXfZtWtqRV+q6usAg6kKPpoTrKx4khLn7d1uFNXooURZoMC0pb lgdBdMZ6KyaTCh+MBRvTCJt8QUbVFcXLEYM6j/hlEXGYEx9bpvg/hrLkIDrjN+unx4 7fLZ6uG6Df0dw== Date: Fri, 20 Oct 2023 11:09:09 -0600 From: Keith Busch To: Bart Van Assche Cc: Christoph Hellwig , Jens Axboe , linux-block@vger.kernel.org, "Martin K . Petersen" , Ming Lei , Damien Le Moal , Yu Kuai , Ed Tsai Subject: Re: [PATCH] block: Improve shared tag set performance Message-ID: References: <20231018180056.2151711-1-bvanassche@acm.org> <20231020044159.GB11984@lst.de> <0d2dce2a-8e01-45d6-b61b-f76493d55863@acm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Fri, Oct 20, 2023 at 09:45:53AM -0700, Bart Van Assche wrote: > > On 10/20/23 09:25, Keith Busch wrote: > > The legacy block request layer didn't have a tag resource shared among > > multiple request queues. Each queue had their own mempool for allocating > > requests. The mempool, I think, would always guarantee everyone could > > get at least one request. > > I think that the above is irrelevant in this context. As an example, SCSI > devices have always shared a pool of tags across multiple logical > units. This behavior has not been changed by the conversion of the > SCSI core from the legacy block layer to blk-mq. > > For other (hardware) block devices it didn't matter either that there > was no upper limit to the number of requests the legacy block layer > could allocate. All hardware block devices I know support fixed size > queues for queuing requests to the block device. I am not sure I understand your point. Those lower layers always were able to get at least one request per request_queue. They can do whatever they want with it after that. This change removes that guarantee for blk-mq in some cases, right? I just don't think you can readily conclude that is "safe" by appealing to the legacy behavior, that's all.