From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-x532.google.com (mail-ed1-x532.google.com [IPv6:2a00:1450:4864:20::532]) by sourceware.org (Postfix) with ESMTPS id DEC493858C20; Tue, 11 Oct 2022 11:34:52 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org DEC493858C20 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-ed1-x532.google.com with SMTP id a13so19726567edj.0; Tue, 11 Oct 2022 04:34:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=ZdiGrTw9/CA65TrmLMxR7tTfPoo7isJIYcR4CepjvAs=; b=U6TfIlDEzsahTBv0MHnsW95YySqJsVFYHqLYgapsfYKU80nFZP/+4bJS9BcfU1viSo WerWxZF1CO0NHjNqIB7WY3XxPmA3NUTkSP7NI1Oscr0hUtPqIV+EHj8WInSGigrX4IdU VDDZndC+HnjaRBF5mOASK6K5CQ7XiXclkP/NsUNIdlYIT61l5Bwzwv3VN/DoqRP5CBp5 uuNZzO3LXpnYe/L+S+bJh2651iUEV6/yRqD0ONVHH1lKqcx0W1G22tmQWp1qCbUkE4xa 9pG1dVfMp+S4x/QyiRW7Tu+EOjEjq8F1EdMHXKRf4Aq8AFy2HU4FjrtBRwxyWGwfTB49 eaIg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=ZdiGrTw9/CA65TrmLMxR7tTfPoo7isJIYcR4CepjvAs=; b=nPjIyCvHN0ynzxdqoqK70Osn0H/IijwM1AiYyCL3Pe+SItQmzX0yarhlp/ekgXJGZ1 AYZm1ZypHpGyod4EB4ba6QlmMn6CQSTKBaHyibcdotfUsrcrK4Kc6pquF4dH3//J52U+ ZzM3EIOlhjK3YeScJFIBAHV8rvsISdX0GZG876xFa8Xyc1+jVpvp7HfulKYoaj+IF96f 66pmz3abspaFXMRpUg0amlVVL1/c20kRKZPw1n1tUMfJkn41ThlNNf6lOEDW/S5YHa+g fb8PyRHBKv2Q2YYOvZ+ii5VDqZYiNNYzLv21DjdF4OztObLLtCeO7C3THfPzKmHYChJN fAKw== X-Gm-Message-State: ACrzQf30s4ft/eULTrXGRc1YN2EX56KTnIMKXWpspMKVyLDGfFamLo8U NExN4c/2+RmAdSjHnhigKHXFzR9gFuixfvBDIxs= X-Google-Smtp-Source: AMsMyM4h2jTyjVY6/wG6cEwp7hAu/uBhzGs4N/clP9HBn11i9X/+1gppLPJ5vkhvysvW45K0/4obGMMHWyO0Fziiz6s= X-Received: by 2002:a05:6402:27c9:b0:45c:3c77:8881 with SMTP id c9-20020a05640227c900b0045c3c778881mr6878499ede.250.1665488091561; Tue, 11 Oct 2022 04:34:51 -0700 (PDT) MIME-Version: 1.0 References: <20221007155452.1299670-1-jwakely@redhat.com> In-Reply-To: From: Richard Biener Date: Tue, 11 Oct 2022 13:34:38 +0200 Message-ID: Subject: Re: [PATCH] libstdc++: Allow emergency EH alloc pool size to be tuned [PR68606] To: Jonathan Wakely Cc: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Tue, Oct 11, 2022 at 1:06 PM Jonathan Wakely wrote: > > On Tue, 11 Oct 2022 at 07:41, Richard Biener wrote: > > > > On Mon, Oct 10, 2022 at 5:10 PM Jonathan Wakely wrote: > > > > > > On Mon, 10 Oct 2022 at 12:17, Jonathan Wakely wrote: > > > > > > > > On Mon, 10 Oct 2022 at 07:18, Richard Biener wrote: > > > > > > > > > > On Fri, Oct 7, 2022 at 5:55 PM Jonathan Wakely via Gcc-patches > > > > > wrote: > > > > > > > > > > > > This needs a little more documentation (see the TODO in the manual), > > > > > > rather than just the comments in the source. This isn't final, but I > > > > > > think it's the direction I want to take. > > > > > > > > > > > > -- >8 -- > > > > > > > > > > > > Implement a long-standing request to support tuning the size of the > > > > > > emergency buffer for allocating exceptions after malloc fails, or to > > > > > > disable that buffer entirely. > > > > > > > > > > > > It's now possible to disable the dynamic allocation of the buffer and > > > > > > use a fixed-size static buffer, via --enable-libstdcxx-static-eh-pool. > > > > > > This is a built-time choice that is baked into libstdc++ and so affects > > > > > > all code linked against that build of libstdc++. > > > > > > > > > > > > The size of the pool can be set by --with-libstdcxx-eh-pool-obj-count=N > > > > > > which is measured in units of sizeof(void*) not bytes. A given exception > > > > > > type such as std::system_error depends on the target, so giving a size > > > > > > in bytes wouldn't be portable across 16/32/64-bit targets. > > > > > > > > > > > > When libstdc++ is configured to use a dynamic buffer, the size of that > > > > > > buffer can now be tuned at runtime by setting the GLIBCXX_TUNABLES > > > > > > environment variable (c.f. PR libstdc++/88264). The number of exceptions > > > > > > to reserve space for is controlled by the "glibcxx.eh_pool.obj_count" > > > > > > and "glibcxx.eh_pool.obj_size" tunables. The pool will be sized to be > > > > > > able to allocate obj_count exceptions of size obj_size*sizeof(void*) and > > > > > > obj_count "dependent" exceptions rethrown by std::rethrow_exception. > > > > > > > > > > > > With the ability to tune the buffer size, we can reduce the default pool > > > > > > size. Most users never need to throw 1kB exceptions in parallel from > > > > > > hundreds of threads after malloc is OOM. > > > > > > > > > > But does it hurt? Back in time when I reworked the allocator to be less > > > > > wasteful the whole point was to allow more exceptions to be in-flight > > > > > during OOM shutdown of a process with many threads. > > > > > > > > It certainly hurts for small systems, but maybe we can keep the large > > > > allocation for 64-bit targets (currently 73kB) and only reduce it for > > > > 32-bit (19kB) and 16-bit (3kB IIRC) targets. > > > > > > Maybe this incremental diff would be an improvement: > > > > > > @@ -90,7 +90,7 @@ using namespace __cxxabiv1; > > > // Assume that the number of concurrent exception objects scales with the > > > // processor word size, i.e., 16-bit systems are not likely to have hundreds > > > // of threads all simultaneously throwing on OOM conditions. > > > -# define EMERGENCY_OBJ_COUNT (8 * __SIZEOF_POINTER__) > > > +# define EMERGENCY_OBJ_COUNT (4 * __SIZEOF_POINTER__ * __SIZEOF_POINTER__) > > > # define MAX_OBJ_COUNT (16 << __SIZEOF_POINTER__) > > > #else > > > # define EMERGENCY_OBJ_COUNT 4 > > > > > > This makes it quadratic in the word size, so on 64-bit targets we'd > > > have space for 256 "reasonable size" exceptions (and twice as many > > > single word exceptions like std::bad_alloc), but only 64 on 32-bit > > > targets, and only 16 on 16-bit ones. > > > > So can we then commonize some of the #defines by using sizeof(void *) > > (taking pointer size as word size?) > > What did you have in mind? Do you mean use sizeof(void*) instead of > the SIZEOF macro? I was just confused and didn't see you commonized EMERGENCY_OBJ_SIZE already, so ignore my comment. > > MAX_OBJ_COUNT uses the SIZEOF macro so it can be used in a > preprocessor condition: > > #ifdef _GLIBCXX_EH_POOL_NOBJS > # if _GLIBCXX_EH_POOL_NOBJS > MAX_OBJ_COUNT >