From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rich Altmaier Date: Fri, 22 Jun 2001 22:50:39 +0000 Subject: Re: [Linux-ia64] IO/TLB bounce buffer space Message-Id: List-Id: References: In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org This is just a note to comment on the mention of: Thus, worst case buffer usage would be 8kB * 253 * 16 * 4 = 126 MB only for this driver! Of course it is highly unlikely that all these buffers will be in use simultaneously. In my experience, the question of the peak demand a system may experience must be governed by 1. what you, the supplier, spec as the peak demand it supports, 2. what the customer understands is the max load the system can be utilized to perform, and have paid to obtain. 3. and any shortfall in actual delivered performance from your spec you can be sure the customer will demand you the supplier pay to make up the difference! This leaves no room for statistical analysis! The customer will not view that using all the resources at once is unlikely and not expected. If you spec 4 controllers of 32 devices each of 253 buffers needed for their peak operation, then the system better deliver this. Or you should change your spec and reduce the numbers. Please don't design for 4 controllers of 32 devices that just limp along. Either they run at full performance all the time, or reduce the numbers to what can be supported! How can the customer, who lays out the dollars for 4*32 devices, interpret this otherwise? Of course you can also take an approach of specing a max aggregate bandwidth, to be "fairly" allocated across all devices. This is another approach that can work, although you will spend a year or so getting "fairly" to satisfy all customers. Better to pull down your config until peak really works. Thanks, Rich Rich Altmaier SGI