<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><blockquote type="cite" class=""><div dir="auto" class="">Eadline's Law : Cache is only good the second time.</div></blockquote><div class=""><br class=""></div>Hmm, that’s why they have all those clever pre-fetchers which try to guess your memory access patterns and predict what's going to be needed next.<div class="">(Your choice whether you read “clever” in a cynical voice or not :-))<br class=""><div class="">*IF* that works, then the cache is useful the first time.</div><div class="">If not, then they can mess things up royally by evicting stuff that you did want there.<br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On 19 Sep 2021, at 12:02, John Hearns <<a href="mailto:hearnsj@gmail.com" class="">hearnsj@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="auto" class="">Eadline's Law : Cache is only good the second time.</div><br class=""><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, 17 Sep 2021, 21:25 Douglas Eadline, <<a href="mailto:deadline@eadline.org" class="">deadline@eadline.org</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">--snip--<br class="">
><br class="">
> Where I disagree with you is (3). Whether or not cache size is important<br class="">
> depends on the size of the job. If your iterating through data-parallel<br class="">
> loops over a large dataset that exceeds cache size, the opportunity to<br class="">
> reread cached data is probably limited or nonexistent. As we often say<br class="">
> here, "it depends". I'm sore someone with better low-level hardware<br class="">
> knowledge will pipe in and tell me why I'm wrong (Cunningham's Law).<br class="">
><br class="">
<br class="">
Of course it all depends. However, as core counts go up, a<br class="">
fixed amount of cache must get shared. Since the high core counts<br class="">
are putting pressure on main memory BW, cache gets more<br class="">
important. This is why AMD is doing V-cache for new processors.<br class="">
Core counts have outstripped memory BW, their solution<br class="">
seems to be big caches. And, cache is only good the second time :-)<br class="">
<br class="">
<br class="">
-- big snip--<br class="">
<br class="">
-- <br class="">
Doug<br class="">
<br class="">
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